- Add ring_buffer memory mappings
The tracing ring buffer was created based on being mostly used with the
splice system call. It is broken up into page ordered sub-buffers and the
reader swaps a new sub-buffer with an existing sub-buffer that's part
of the write buffer. It then has total access to the swapped out
sub-buffer and can do copyless movements of the memory into other mediums
(file system, network, etc).
The buffer is great for passing around the ring buffer contents in the
kernel, but is not so good for when the consumer is the user space task
itself.
A new interface is added that allows user space to memory map the ring
buffer. It will get all the write sub-buffers as well as reader sub-buffer
(that is not written to). It can send an ioctl to change which sub-buffer
is the new reader sub-buffer.
The ring buffer is read only to user space. It only needs to call the
ioctl when it is finished with a sub-buffer and needs a new sub-buffer
that the writer will not write over.
A self test program was also created for testing and can be used as
an example for the interface to user space. The libtracefs (external
to the kernel) also has code that interacts with this, although it is
disabled until the interface is in a official release. It can be enabled
by compiling the library with a special flag. This was used for testing
applications that perform better with the buffer being mapped.
Memory mapped buffers have limitations. The main one is that it can not be
used with the snapshot logic. If the buffer is mapped, snapshots will be
disabled. If any logic is set to trigger snapshots on a buffer, that
buffer will not be allowed to be mapped.
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Merge tag 'trace-ringbuffer-v6.10' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace
Pull tracing ring buffer updates from Steven Rostedt:
"Add ring_buffer memory mappings.
The tracing ring buffer was created based on being mostly used with
the splice system call. It is broken up into page ordered sub-buffers
and the reader swaps a new sub-buffer with an existing sub-buffer
that's part of the write buffer. It then has total access to the
swapped out sub-buffer and can do copyless movements of the memory
into other mediums (file system, network, etc).
The buffer is great for passing around the ring buffer contents in the
kernel, but is not so good for when the consumer is the user space
task itself.
A new interface is added that allows user space to memory map the ring
buffer. It will get all the write sub-buffers as well as reader
sub-buffer (that is not written to). It can send an ioctl to change
which sub-buffer is the new reader sub-buffer.
The ring buffer is read only to user space. It only needs to call the
ioctl when it is finished with a sub-buffer and needs a new sub-buffer
that the writer will not write over.
A self test program was also created for testing and can be used as an
example for the interface to user space. The libtracefs (external to
the kernel) also has code that interacts with this, although it is
disabled until the interface is in a official release. It can be
enabled by compiling the library with a special flag. This was used
for testing applications that perform better with the buffer being
mapped.
Memory mapped buffers have limitations. The main one is that it can
not be used with the snapshot logic. If the buffer is mapped,
snapshots will be disabled. If any logic is set to trigger snapshots
on a buffer, that buffer will not be allowed to be mapped"
* tag 'trace-ringbuffer-v6.10' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
ring-buffer: Add cast to unsigned long addr passed to virt_to_page()
ring-buffer: Have mmapped ring buffer keep track of missed events
ring-buffer/selftest: Add ring-buffer mapping test
Documentation: tracing: Add ring-buffer mapping
tracing: Allow user-space mapping of the ring-buffer
ring-buffer: Introducing ring-buffer mapping functions
ring-buffer: Allocate sub-buffers with __GFP_COMP
53683e4080