Hi Vincent, On Thu, 11 Jan 2024 16:17:11 +0000 Vincent Donnefort <vdonnef...@google.com> 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 <vdonnef...@google.com> > > 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 <vdonnef...@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) <mhira...@kernel.org>