495 lines
21 KiB
Plaintext
495 lines
21 KiB
Plaintext
relay interface (formerly relayfs)
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==================================
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The relay interface provides a means for kernel applications to
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efficiently log and transfer large quantities of data from the kernel
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to userspace via user-defined 'relay channels'.
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A 'relay channel' is a kernel->user data relay mechanism implemented
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as a set of per-cpu kernel buffers ('channel buffers'), each
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represented as a regular file ('relay file') in user space. Kernel
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clients write into the channel buffers using efficient write
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functions; these automatically log into the current cpu's channel
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buffer. User space applications mmap() or read() from the relay files
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and retrieve the data as it becomes available. The relay files
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themselves are files created in a host filesystem, e.g. debugfs, and
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are associated with the channel buffers using the API described below.
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The format of the data logged into the channel buffers is completely
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up to the kernel client; the relay interface does however provide
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hooks which allow kernel clients to impose some structure on the
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buffer data. The relay interface doesn't implement any form of data
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filtering - this also is left to the kernel client. The purpose is to
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keep things as simple as possible.
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This document provides an overview of the relay interface API. The
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details of the function parameters are documented along with the
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functions in the relay interface code - please see that for details.
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Semantics
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=========
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Each relay channel has one buffer per CPU, each buffer has one or more
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sub-buffers. Messages are written to the first sub-buffer until it is
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too full to contain a new message, in which case it it is written to
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the next (if available). Messages are never split across sub-buffers.
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At this point, userspace can be notified so it empties the first
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sub-buffer, while the kernel continues writing to the next.
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When notified that a sub-buffer is full, the kernel knows how many
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bytes of it are padding i.e. unused space occurring because a complete
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message couldn't fit into a sub-buffer. Userspace can use this
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knowledge to copy only valid data.
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After copying it, userspace can notify the kernel that a sub-buffer
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has been consumed.
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A relay channel can operate in a mode where it will overwrite data not
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yet collected by userspace, and not wait for it to be consumed.
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The relay channel itself does not provide for communication of such
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data between userspace and kernel, allowing the kernel side to remain
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simple and not impose a single interface on userspace. It does
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provide a set of examples and a separate helper though, described
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below.
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The read() interface both removes padding and internally consumes the
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read sub-buffers; thus in cases where read(2) is being used to drain
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the channel buffers, special-purpose communication between kernel and
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user isn't necessary for basic operation.
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One of the major goals of the relay interface is to provide a low
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overhead mechanism for conveying kernel data to userspace. While the
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read() interface is easy to use, it's not as efficient as the mmap()
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approach; the example code attempts to make the tradeoff between the
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two approaches as small as possible.
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klog and relay-apps example code
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================================
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The relay interface itself is ready to use, but to make things easier,
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a couple simple utility functions and a set of examples are provided.
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The relay-apps example tarball, available on the relay sourceforge
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site, contains a set of self-contained examples, each consisting of a
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pair of .c files containing boilerplate code for each of the user and
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kernel sides of a relay application. When combined these two sets of
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boilerplate code provide glue to easily stream data to disk, without
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having to bother with mundane housekeeping chores.
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The 'klog debugging functions' patch (klog.patch in the relay-apps
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tarball) provides a couple of high-level logging functions to the
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kernel which allow writing formatted text or raw data to a channel,
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regardless of whether a channel to write into exists or not, or even
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whether the relay interface is compiled into the kernel or not. These
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functions allow you to put unconditional 'trace' statements anywhere
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in the kernel or kernel modules; only when there is a 'klog handler'
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registered will data actually be logged (see the klog and kleak
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examples for details).
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It is of course possible to use the relay interface from scratch,
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i.e. without using any of the relay-apps example code or klog, but
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you'll have to implement communication between userspace and kernel,
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allowing both to convey the state of buffers (full, empty, amount of
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padding). The read() interface both removes padding and internally
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consumes the read sub-buffers; thus in cases where read(2) is being
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used to drain the channel buffers, special-purpose communication
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between kernel and user isn't necessary for basic operation. Things
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such as buffer-full conditions would still need to be communicated via
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some channel though.
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klog and the relay-apps examples can be found in the relay-apps
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tarball on http://relayfs.sourceforge.net
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The relay interface user space API
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==================================
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The relay interface implements basic file operations for user space
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access to relay channel buffer data. Here are the file operations
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that are available and some comments regarding their behavior:
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open() enables user to open an _existing_ channel buffer.
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mmap() results in channel buffer being mapped into the caller's
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memory space. Note that you can't do a partial mmap - you
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must map the entire file, which is NRBUF * SUBBUFSIZE.
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read() read the contents of a channel buffer. The bytes read are
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'consumed' by the reader, i.e. they won't be available
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again to subsequent reads. If the channel is being used
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in no-overwrite mode (the default), it can be read at any
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time even if there's an active kernel writer. If the
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channel is being used in overwrite mode and there are
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active channel writers, results may be unpredictable -
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users should make sure that all logging to the channel has
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ended before using read() with overwrite mode. Sub-buffer
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padding is automatically removed and will not be seen by
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the reader.
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sendfile() transfer data from a channel buffer to an output file
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descriptor. Sub-buffer padding is automatically removed
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and will not be seen by the reader.
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poll() POLLIN/POLLRDNORM/POLLERR supported. User applications are
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notified when sub-buffer boundaries are crossed.
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close() decrements the channel buffer's refcount. When the refcount
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reaches 0, i.e. when no process or kernel client has the
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buffer open, the channel buffer is freed.
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In order for a user application to make use of relay files, the
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host filesystem must be mounted. For example,
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mount -t debugfs debugfs /sys/kernel/debug
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NOTE: the host filesystem doesn't need to be mounted for kernel
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clients to create or use channels - it only needs to be
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mounted when user space applications need access to the buffer
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data.
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The relay interface kernel API
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==============================
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Here's a summary of the API the relay interface provides to in-kernel clients:
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TBD(curr. line MT:/API/)
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channel management functions:
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relay_open(base_filename, parent, subbuf_size, n_subbufs,
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callbacks, private_data)
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relay_close(chan)
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relay_flush(chan)
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relay_reset(chan)
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channel management typically called on instigation of userspace:
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relay_subbufs_consumed(chan, cpu, subbufs_consumed)
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write functions:
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relay_write(chan, data, length)
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__relay_write(chan, data, length)
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relay_reserve(chan, length)
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callbacks:
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subbuf_start(buf, subbuf, prev_subbuf, prev_padding)
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buf_mapped(buf, filp)
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buf_unmapped(buf, filp)
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create_buf_file(filename, parent, mode, buf, is_global)
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remove_buf_file(dentry)
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helper functions:
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relay_buf_full(buf)
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subbuf_start_reserve(buf, length)
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Creating a channel
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------------------
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relay_open() is used to create a channel, along with its per-cpu
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channel buffers. Each channel buffer will have an associated file
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created for it in the host filesystem, which can be and mmapped or
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read from in user space. The files are named basename0...basenameN-1
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where N is the number of online cpus, and by default will be created
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in the root of the filesystem (if the parent param is NULL). If you
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want a directory structure to contain your relay files, you should
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create it using the host filesystem's directory creation function,
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e.g. debugfs_create_dir(), and pass the parent directory to
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relay_open(). Users are responsible for cleaning up any directory
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structure they create, when the channel is closed - again the host
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filesystem's directory removal functions should be used for that,
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e.g. debugfs_remove().
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In order for a channel to be created and the host filesystem's files
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associated with its channel buffers, the user must provide definitions
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for two callback functions, create_buf_file() and remove_buf_file().
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create_buf_file() is called once for each per-cpu buffer from
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relay_open() and allows the user to create the file which will be used
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to represent the corresponding channel buffer. The callback should
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return the dentry of the file created to represent the channel buffer.
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remove_buf_file() must also be defined; it's responsible for deleting
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the file(s) created in create_buf_file() and is called during
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relay_close().
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Here are some typical definitions for these callbacks, in this case
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using debugfs:
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/*
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* create_buf_file() callback. Creates relay file in debugfs.
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*/
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static struct dentry *create_buf_file_handler(const char *filename,
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struct dentry *parent,
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int mode,
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struct rchan_buf *buf,
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int *is_global)
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{
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return debugfs_create_file(filename, mode, parent, buf,
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&relay_file_operations);
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}
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/*
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* remove_buf_file() callback. Removes relay file from debugfs.
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*/
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static int remove_buf_file_handler(struct dentry *dentry)
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{
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debugfs_remove(dentry);
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return 0;
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}
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/*
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* relay interface callbacks
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*/
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static struct rchan_callbacks relay_callbacks =
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{
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.create_buf_file = create_buf_file_handler,
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.remove_buf_file = remove_buf_file_handler,
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};
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And an example relay_open() invocation using them:
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chan = relay_open("cpu", NULL, SUBBUF_SIZE, N_SUBBUFS, &relay_callbacks, NULL);
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If the create_buf_file() callback fails, or isn't defined, channel
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creation and thus relay_open() will fail.
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The total size of each per-cpu buffer is calculated by multiplying the
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number of sub-buffers by the sub-buffer size passed into relay_open().
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The idea behind sub-buffers is that they're basically an extension of
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double-buffering to N buffers, and they also allow applications to
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easily implement random-access-on-buffer-boundary schemes, which can
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be important for some high-volume applications. The number and size
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of sub-buffers is completely dependent on the application and even for
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the same application, different conditions will warrant different
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values for these parameters at different times. Typically, the right
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values to use are best decided after some experimentation; in general,
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though, it's safe to assume that having only 1 sub-buffer is a bad
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idea - you're guaranteed to either overwrite data or lose events
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depending on the channel mode being used.
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The create_buf_file() implementation can also be defined in such a way
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as to allow the creation of a single 'global' buffer instead of the
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default per-cpu set. This can be useful for applications interested
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mainly in seeing the relative ordering of system-wide events without
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the need to bother with saving explicit timestamps for the purpose of
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merging/sorting per-cpu files in a postprocessing step.
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To have relay_open() create a global buffer, the create_buf_file()
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implementation should set the value of the is_global outparam to a
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non-zero value in addition to creating the file that will be used to
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represent the single buffer. In the case of a global buffer,
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create_buf_file() and remove_buf_file() will be called only once. The
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normal channel-writing functions, e.g. relay_write(), can still be
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used - writes from any cpu will transparently end up in the global
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buffer - but since it is a global buffer, callers should make sure
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they use the proper locking for such a buffer, either by wrapping
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writes in a spinlock, or by copying a write function from relay.h and
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creating a local version that internally does the proper locking.
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The private_data passed into relay_open() allows clients to associate
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user-defined data with a channel, and is immediately available
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(including in create_buf_file()) via chan->private_data or
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buf->chan->private_data.
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Buffer-only channels
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--------------------
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These channels have no files associated and can be created with
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relay_open(NULL, NULL, ...). Such channels are useful in scenarios such
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as when doing early tracing in the kernel, before the VFS is up. In these
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cases, one may open a buffer-only channel and then call
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relay_late_setup_files() when the kernel is ready to handle files,
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to expose the buffered data to the userspace.
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Channel 'modes'
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---------------
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relay channels can be used in either of two modes - 'overwrite' or
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'no-overwrite'. The mode is entirely determined by the implementation
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of the subbuf_start() callback, as described below. The default if no
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subbuf_start() callback is defined is 'no-overwrite' mode. If the
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default mode suits your needs, and you plan to use the read()
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interface to retrieve channel data, you can ignore the details of this
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section, as it pertains mainly to mmap() implementations.
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In 'overwrite' mode, also known as 'flight recorder' mode, writes
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continuously cycle around the buffer and will never fail, but will
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unconditionally overwrite old data regardless of whether it's actually
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been consumed. In no-overwrite mode, writes will fail, i.e. data will
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be lost, if the number of unconsumed sub-buffers equals the total
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number of sub-buffers in the channel. It should be clear that if
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there is no consumer or if the consumer can't consume sub-buffers fast
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enough, data will be lost in either case; the only difference is
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whether data is lost from the beginning or the end of a buffer.
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As explained above, a relay channel is made of up one or more
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per-cpu channel buffers, each implemented as a circular buffer
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subdivided into one or more sub-buffers. Messages are written into
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the current sub-buffer of the channel's current per-cpu buffer via the
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write functions described below. Whenever a message can't fit into
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the current sub-buffer, because there's no room left for it, the
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client is notified via the subbuf_start() callback that a switch to a
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new sub-buffer is about to occur. The client uses this callback to 1)
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initialize the next sub-buffer if appropriate 2) finalize the previous
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sub-buffer if appropriate and 3) return a boolean value indicating
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whether or not to actually move on to the next sub-buffer.
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To implement 'no-overwrite' mode, the userspace client would provide
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an implementation of the subbuf_start() callback something like the
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following:
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static int subbuf_start(struct rchan_buf *buf,
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void *subbuf,
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void *prev_subbuf,
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unsigned int prev_padding)
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{
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if (prev_subbuf)
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*((unsigned *)prev_subbuf) = prev_padding;
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if (relay_buf_full(buf))
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return 0;
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subbuf_start_reserve(buf, sizeof(unsigned int));
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return 1;
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}
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If the current buffer is full, i.e. all sub-buffers remain unconsumed,
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the callback returns 0 to indicate that the buffer switch should not
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occur yet, i.e. until the consumer has had a chance to read the
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current set of ready sub-buffers. For the relay_buf_full() function
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to make sense, the consumer is responsible for notifying the relay
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interface when sub-buffers have been consumed via
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relay_subbufs_consumed(). Any subsequent attempts to write into the
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buffer will again invoke the subbuf_start() callback with the same
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parameters; only when the consumer has consumed one or more of the
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ready sub-buffers will relay_buf_full() return 0, in which case the
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buffer switch can continue.
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The implementation of the subbuf_start() callback for 'overwrite' mode
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would be very similar:
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static int subbuf_start(struct rchan_buf *buf,
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void *subbuf,
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void *prev_subbuf,
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unsigned int prev_padding)
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{
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if (prev_subbuf)
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*((unsigned *)prev_subbuf) = prev_padding;
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subbuf_start_reserve(buf, sizeof(unsigned int));
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return 1;
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}
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In this case, the relay_buf_full() check is meaningless and the
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callback always returns 1, causing the buffer switch to occur
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unconditionally. It's also meaningless for the client to use the
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relay_subbufs_consumed() function in this mode, as it's never
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consulted.
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The default subbuf_start() implementation, used if the client doesn't
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define any callbacks, or doesn't define the subbuf_start() callback,
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implements the simplest possible 'no-overwrite' mode, i.e. it does
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nothing but return 0.
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Header information can be reserved at the beginning of each sub-buffer
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by calling the subbuf_start_reserve() helper function from within the
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subbuf_start() callback. This reserved area can be used to store
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whatever information the client wants. In the example above, room is
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reserved in each sub-buffer to store the padding count for that
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sub-buffer. This is filled in for the previous sub-buffer in the
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subbuf_start() implementation; the padding value for the previous
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sub-buffer is passed into the subbuf_start() callback along with a
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pointer to the previous sub-buffer, since the padding value isn't
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known until a sub-buffer is filled. The subbuf_start() callback is
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also called for the first sub-buffer when the channel is opened, to
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give the client a chance to reserve space in it. In this case the
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previous sub-buffer pointer passed into the callback will be NULL, so
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the client should check the value of the prev_subbuf pointer before
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writing into the previous sub-buffer.
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Writing to a channel
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--------------------
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Kernel clients write data into the current cpu's channel buffer using
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relay_write() or __relay_write(). relay_write() is the main logging
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function - it uses local_irqsave() to protect the buffer and should be
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used if you might be logging from interrupt context. If you know
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you'll never be logging from interrupt context, you can use
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__relay_write(), which only disables preemption. These functions
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don't return a value, so you can't determine whether or not they
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failed - the assumption is that you wouldn't want to check a return
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value in the fast logging path anyway, and that they'll always succeed
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unless the buffer is full and no-overwrite mode is being used, in
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which case you can detect a failed write in the subbuf_start()
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callback by calling the relay_buf_full() helper function.
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relay_reserve() is used to reserve a slot in a channel buffer which
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can be written to later. This would typically be used in applications
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that need to write directly into a channel buffer without having to
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stage data in a temporary buffer beforehand. Because the actual write
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may not happen immediately after the slot is reserved, applications
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using relay_reserve() can keep a count of the number of bytes actually
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written, either in space reserved in the sub-buffers themselves or as
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a separate array. See the 'reserve' example in the relay-apps tarball
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at http://relayfs.sourceforge.net for an example of how this can be
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done. Because the write is under control of the client and is
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separated from the reserve, relay_reserve() doesn't protect the buffer
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at all - it's up to the client to provide the appropriate
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synchronization when using relay_reserve().
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Closing a channel
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-----------------
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The client calls relay_close() when it's finished using the channel.
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The channel and its associated buffers are destroyed when there are no
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longer any references to any of the channel buffers. relay_flush()
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forces a sub-buffer switch on all the channel buffers, and can be used
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to finalize and process the last sub-buffers before the channel is
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closed.
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Misc
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----
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Some applications may want to keep a channel around and re-use it
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rather than open and close a new channel for each use. relay_reset()
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can be used for this purpose - it resets a channel to its initial
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state without reallocating channel buffer memory or destroying
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existing mappings. It should however only be called when it's safe to
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do so, i.e. when the channel isn't currently being written to.
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Finally, there are a couple of utility callbacks that can be used for
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different purposes. buf_mapped() is called whenever a channel buffer
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is mmapped from user space and buf_unmapped() is called when it's
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unmapped. The client can use this notification to trigger actions
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within the kernel application, such as enabling/disabling logging to
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the channel.
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Resources
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=========
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For news, example code, mailing list, etc. see the relay interface homepage:
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http://relayfs.sourceforge.net
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Credits
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=======
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The ideas and specs for the relay interface came about as a result of
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discussions on tracing involving the following:
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Michel Dagenais <michel.dagenais@polymtl.ca>
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Richard Moore <richardj_moore@uk.ibm.com>
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Bob Wisniewski <bob@watson.ibm.com>
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Karim Yaghmour <karim@opersys.com>
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Tom Zanussi <zanussi@us.ibm.com>
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Also thanks to Hubertus Franke for a lot of useful suggestions and bug
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reports.
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