Documentation / filesystems / relay.rst

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.. SPDX-License-Identifier: GPL-2.0

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.


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 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

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

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

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)

  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)


    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)

  helper functions::

    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

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,
						umode_t mode,
						struct rchan_buf *buf,
						int *is_global)
	    return debugfs_create_file(filename, mode, parent, buf,

    * remove_buf_file() callback.  Removes relay file from debugfs.
    static int remove_buf_file_handler(struct dentry *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

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

    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,
			    size_t 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

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 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


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.


For news, example code, mailing list, etc. see the relay interface homepage:


The ideas and specs for the relay interface 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