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diff --git a/Documentation/filesystems/relayfs.txt b/Documentation/filesystems/relayfs.txt new file mode 100644 index 000000000000..d803abed29f0 --- /dev/null +++ b/Documentation/filesystems/relayfs.txt @@ -0,0 +1,362 @@ + +relayfs - a high-speed data relay filesystem +============================================ + +relayfs is a filesystem designed to provide an efficient mechanism for +tools and facilities to relay large and potentially sustained streams +of data from kernel space to user space. + +The main abstraction of relayfs is the 'channel'. A channel consists +of a set of per-cpu kernel buffers each represented by a file in the +relayfs filesystem. Kernel clients write into a channel using +efficient write functions which automatically log to the current cpu's +channel buffer. User space applications mmap() the per-cpu files and +retrieve the data as it becomes available. + +The format of the data logged into the channel buffers is completely +up to the relayfs client; relayfs does however provide hooks which +allow clients to impose some structure on the buffer data. Nor does +relayfs implement any form of data filtering - this also is left to +the client. The purpose is to keep relayfs as simple as possible. + +This document provides an overview of the relayfs API. The details of +the function parameters are documented along with the functions in the +filesystem code - please see that for details. + +Semantics +========= + +Each relayfs 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. 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. + +relayfs can operate in a mode where it will overwrite data not yet +collected by userspace, and not wait for it to consume it. + +relayfs 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 separate +helper though, described below. + +klog, relay-app & librelay +========================== + +relayfs itself is ready to use, but to make things easier, two +additional systems are provided. klog is a simple wrapper to make +writing formatted text or raw data to a channel simpler, regardless of +whether a channel to write into exists or not, or whether relayfs is +compiled into the kernel or is configured as a module. relay-app is +the kernel counterpart of userspace librelay.c, combined these two +files provide glue to easily stream data to disk, without having to +bother with housekeeping. klog and relay-app can be used together, +with klog providing high-level logging functions to the kernel and +relay-app taking care of kernel-user control and disk-logging chores. + +It is possible to use relayfs without relay-app & librelay, but you'll +have to implement communication between userspace and kernel, allowing +both to convey the state of buffers (full, empty, amount of padding). + +klog, relay-app and librelay can be found in the relay-apps tarball on +http://relayfs.sourceforge.net + +The relayfs user space API +========================== + +relayfs implements basic file operations for user space access to +relayfs channel buffer data. Here are the file operations that are +available and some comments regarding their behavior: + +open() enables user to open an _existing_ 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. + +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 relayfs files, the +relayfs filesystem must be mounted. For example, + + mount -t relayfs relayfs /mnt/relay + +NOTE: relayfs 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 relayfs kernel API +====================== + +Here's a summary of the API relayfs provides to in-kernel clients: + + + channel management functions: + + relay_open(base_filename, parent, subbuf_size, n_subbufs, + callbacks) + relay_close(chan) + relay_flush(chan) + relay_reset(chan) + relayfs_create_dir(name, parent) + relayfs_remove_dir(dentry) + + 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) + + 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 relayfs filesystem, which can be opened and +mmapped from user space if desired. 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 you want a +directory structure to contain your relayfs files, you can create it +with relayfs_create_dir() and pass the parent directory to +relay_open(). Clients are responsible for cleaning up any directory +structure they create when the channel is closed - use +relayfs_remove_dir() for that. + +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. + +Channel 'modes' +--------------- + +relayfs 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. 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 enought, +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 relayfs 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 go ahead with the sub-buffer switch. + +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 reponsible for notifying relayfs 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 relayfs homepage: + + http://relayfs.sourceforge.net + + +Credits +======= + +The ideas and specs for relayfs 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. |