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