363 строки
15 KiB
Plaintext
363 строки
15 KiB
Plaintext
|
|
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.
|