Linux Cross Reference
Linux-2.6.17/Documentation/filesystems/relayfs.txt

   
   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 set of
  examples and a separate helper though, described below.
  
  klog and relay-apps example code
  ================================
  
  relayfs 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 relayfs 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 relayfs application; 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
  whether relayfs is compiled into the kernel or is configured as a
  module.  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 relayfs 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).
  
  klog and the relay-apps examples 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)
     relayfs_create_file(name, parent, mode, fops, data)
     relayfs_remove_file(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)
     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 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.
 
 Creating non-relay files
 ------------------------
 
 relay_open() automatically creates files in the relayfs filesystem to
 represent the per-cpu kernel buffers; it's often useful for
 applications to be able to create their own files alongside the relay
 files in the relayfs filesystem as well e.g. 'control' files much like
 those created in /proc or debugfs for similar purposes, used to
 communicate control information between the kernel and user sides of a
 relayfs application.  For this purpose the relayfs_create_file() and
 relayfs_remove_file() API functions exist.  For relayfs_create_file(),
 the caller passes in a set of user-defined file operations to be used
 for the file and an optional void * to a user-specified data item,
 which will be accessible via inode->u.generic_ip (see the relay-apps
 tarball for examples).  The file_operations are a required parameter
 to relayfs_create_file() and thus the semantics of these files are
 completely defined by the caller.
 
 See the relay-apps tarball at http://relayfs.sourceforge.net for
 examples of how these non-relay files are meant to be used.
 
 Creating relay files in other filesystems
 -----------------------------------------
 
 By default of course, relay_open() creates relay files in the relayfs
 filesystem.  Because relay_file_operations is exported, however, it's
 also possible to create and use relay files in other pseudo-filesytems
 such as debugfs.
 
 For this purpose, two callback functions are provided,
 create_buf_file() and remove_buf_file().  create_buf_file() is called
 once for each per-cpu buffer from relay_open() to allow the client to
 create a file to be used to represent the corresponding buffer; if
 this callback is not defined, the default implementation will create
 and return a file in the relayfs filesystem to represent the buffer.
 The callback should return the dentry of the file created to represent
 the relay buffer.  Note that the parent directory passed to
 relay_open() (and passed along to the callback), if specified, must
 exist in the same filesystem the new relay file is created in.  If
 create_buf_file() is defined, 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().
 
 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 relayfs_fs.h and
 creating a local version that internally does the proper locking.
 
 See the 'exported-relayfile' examples in the relay-apps tarball for
 examples of creating and using relay files in debugfs.
 
 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.