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 stucture 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 Richard Moore Bob Wisniewski Karim Yaghmour Tom Zanussi Also thanks to Hubertus Franke for a lot of useful suggestions and bug reports.