Re: [PATCH v11 4/5] Documentation: tracing: Add ring-buffer mapping

From: Google
Date: Sun Jan 14 2024 - 09:27:07 EST

Hi Vincent,

On Thu, 11 Jan 2024 16:17:11 +0000
Vincent Donnefort <vdonnefort@xxxxxxxxxx> wrote:

> It is now possible to mmap() a ring-buffer to stream its content. Add
> some documentation and a code example.
>
> Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
>
> diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
> index 5092d6c13af5..0b300901fd75 100644
> --- a/Documentation/trace/index.rst
> +++ b/Documentation/trace/index.rst
> @@ -29,6 +29,7 @@ Linux Tracing Technologies
> timerlat-tracer
> intel_th
> ring-buffer-design
> + ring-buffer-map
> stm
> sys-t
> coresight/index
> diff --git a/Documentation/trace/ring-buffer-map.rst b/Documentation/trace/ring-buffer-map.rst
> new file mode 100644
> index 000000000000..2ba7b5339178
> --- /dev/null
> +++ b/Documentation/trace/ring-buffer-map.rst
> @@ -0,0 +1,105 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +==================================
> +Tracefs ring-buffer memory mapping
> +==================================
> +
> +:Author: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
> +
> +Overview
> +========
> +Tracefs ring-buffer memory map provides an efficient method to stream data
> +as no memory copy is necessary. The application mapping the ring-buffer becomes
> +then a consumer for that ring-buffer, in a similar fashion to trace_pipe.
> +
> +Memory mapping setup
> +====================
> +The mapping works with a mmap() of the trace_pipe_raw interface.
> +
> +The first system page of the mapping contains ring-buffer statistics and
> +description. It is referred as the meta-page. One of the most important field of
> +the meta-page is the reader. It contains the subbuf ID which can be safely read
> +by the mapper (see ring-buffer-design.rst).
> +
> +The meta-page is followed by all the subbuf, ordered by ascendant ID. It is
> +therefore effortless to know where the reader starts in the mapping:
> +
> +.. code-block:: c
> +
> + reader_id = meta->reader->id;
> + reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;
> +
> +When the application is done with the current reader, it can get a new one using
> +the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GET_READER. This ioctl also updates
> +the meta-page fields.
> +
> +Limitations
> +===========
> +When a mapping is in place on a Tracefs ring-buffer, it is not possible to
> +either resize it (either by increasing the entire size of the ring-buffer or
> +each subbuf). It is also not possible to use snapshot or splice.

I've played with the sample code.

- "free_buffer" just doesn't work when the process is mmap the ring buffer.
- After mmap the buffers, when the snapshot took, the IOCTL returns an error.

OK, but I rather like to fail snapshot with -EBUSY when the buffer is mmaped.

> +
> +Concurrent readers (either another application mapping that ring-buffer or the
> +kernel with trace_pipe) are allowed but not recommended. They will compete for
> +the ring-buffer and the output is unpredictable.
> +
> +Example
> +=======
> +
> +.. code-block:: c
> +
> + #include <fcntl.h>
> + #include <stdio.h>
> + #include <stdlib.h>
> + #include <unistd.h>
> +
> + #include <linux/trace_mmap.h>
> +
> + #include <sys/mman.h>
> + #include <sys/ioctl.h>
> +
> + #define TRACE_PIPE_RAW "/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw"
> +
> + int main(void)
> + {
> + int page_size = getpagesize(), fd, reader_id;
> + unsigned long meta_len, data_len;
> + struct trace_buffer_meta *meta;
> + void *map, *reader, *data;
> +
> + fd = open(TRACE_PIPE_RAW, O_RDONLY);
> + if (fd < 0)
> + exit(EXIT_FAILURE);
> +
> + map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
> + if (map == MAP_FAILED)
> + exit(EXIT_FAILURE);
> +
> + meta = (struct trace_buffer_meta *)map;
> + meta_len = meta->meta_page_size;
> +
> + printf("entries: %lu\n", meta->entries);
> + printf("overrun: %lu\n", meta->overrun);
> + printf("read: %lu\n", meta->read);
> + printf("subbufs_touched:%lu\n", meta->subbufs_touched);
> + printf("subbufs_lost: %lu\n", meta->subbufs_lost);
> + printf("subbufs_read: %lu\n", meta->subbufs_read);
> + printf("nr_subbufs: %u\n", meta->nr_subbufs);
> +
> + data_len = meta->subbuf_size * meta->nr_subbufs;
> + data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, data_len);
> + if (data == MAP_FAILED)
> + exit(EXIT_FAILURE);
> +
> + if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0)
> + exit(EXIT_FAILURE);
> +
> + reader_id = meta->reader.id;
> + reader = data + meta->subbuf_size * reader_id;

Also, this caused a bus error if I add below 2 lines here.

printf("reader_id: %d, addr: %p\n", reader_id, reader);
printf("read data head: %lx\n", *(unsigned long *)reader);

-----
/ # cd /sys/kernel/tracing/
/sys/kernel/tracing # echo 1 > events/enable
[ 17.941894] Scheduler tracepoints stat_sleep, stat_iowait, stat_blocked and stat_runtime require the kernel parameter schedstats=enable or kernel.sched_schedstats=1
/sys/kernel/tracing #
/sys/kernel/tracing # echo 1 > buffer_percent
/sys/kernel/tracing # /mnt/rbmap2
entries: 245291
overrun: 203741
read: 0
subbufs_touched:2041
subbufs_lost: 1688
subbufs_read: 0
nr_subbufs: 355
reader_id: 1, addr: 0x7f0cde51a000
Bus error
-----

Is this expected behavior? how can I read the ring buffer?

Thank you,

> +
> + munmap(data, data_len);
> + munmap(meta, meta_len);
> + close (fd);
> +
> + return 0;
> + }
> --
> 2.43.0.275.g3460e3d667-goog
>


--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>