Re: [PATCH v11 4/5] Documentation: tracing: Add ring-buffer mapping
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Date: 2024-01-14 14:26:47
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Hi Vincent, On Thu, 11 Jan 2024 16:17:11 +0000 Vincent Donnefort [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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 <redacted>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/indexdiff --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@google.com> + +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) [off-list ref]