The tracing ring-buffers can be stored on disk or sent to network
without any copy via splice. However the later doesn't allow real time
processing of the traces. A solution is to give userspace direct access
to the ring-buffer pages via a mapping. An application can now become a
consumer of the ring-buffer, in a similar fashion to what trace_pipe
offers.
Attached to this cover letter an example of consuming read for a
ring-buffer, using libtracefs.
Vincent
v6 -> v7:
* Rebase onto lore.kernel.org/lkml/20231215175502.106587...@goodmis.org/
* Support for subbufs
* Rename subbufs into bpages
v5 -> v6:
* Rebase on next-20230802.
* (unsigned long) -> (void *) cast for virt_to_page().
* Add a wait for the GET_READER_PAGE ioctl.
* Move writer fields update (overrun/pages_lost/entries/pages_touched)
in the irq_work.
* Rearrange id in struct buffer_page.
* Rearrange the meta-page.
* ring_buffer_meta_page -> trace_buffer_meta_page.
* Add meta_struct_len into the meta-page.
v4 -> v5:
* Trivial rebase onto 6.5-rc3 (previously 6.4-rc3)
v3 -> v4:
* Add to the meta-page:
- pages_lost / pages_read (allow to compute how full is the
ring-buffer)
- read (allow to compute how many entries can be read)
- A reader_page struct.
* Rename ring_buffer_meta_header -> ring_buffer_meta
* Rename ring_buffer_get_reader_page -> ring_buffer_map_get_reader_page
* Properly consume events on ring_buffer_map_get_reader_page() with
rb_advance_reader().
v2 -> v3:
* Remove data page list (for non-consuming read)
** Implies removing order > 0 meta-page
* Add a new meta page field ->read
* Rename ring_buffer_meta_page_header into ring_buffer_meta_header
v1 -> v2:
* Hide data_pages from the userspace struct
* Fix META_PAGE_MAX_PAGES
* Support for order > 0 meta-page
* Add missing page->mapping.
---
/* Need to access private struct to save counters */
struct kbuffer {
unsigned long long timestamp;
long long lost_events;
unsigned long flags;
void *subbuffer;
void *data;
unsigned int index;
unsigned int curr;
unsigned int next;
unsigned int size;
unsigned int start;
unsigned int first;
unsigned int (*read_4)(void *ptr);
unsigned long long (*read_8)(void *ptr);
unsigned long long (*read_long)(struct kbuffer *kbuf, void *ptr);
int (*next_event)(struct kbuffer *kbuf);
};
struct trace_buffer_meta {
unsigned long entries;
unsigned long overrun;
unsigned long read;
unsigned long bpages_touched;
unsigned long bpages_lost;
unsigned long bpages_read;
struct {
unsigned long lost_events; /* Events lost at the time of
the reader swap */
__u32 id; /* Reader bpage ID from 0 to
nr_bpages - 1 */
__u32 read; /* Number of bytes read on the
reader bpage */
} reader;
__u32 bpage_size; /* Size of each buffer page
including the header */
__u32 nr_bpages; /* Number of buffer pages in
the ring-buffer */
__u32 meta_page_size; /* Size of the meta-page */
__u32 meta_struct_len; /* Len of this struct */
};
static char *argv0;
static bool exit_requested;
static char *get_this_name(void)
{
static char *this_name;
char *arg;
char *p;
if (this_name)
return this_name;
arg = argv0;
p = arg+strlen(arg);
while (p >= arg && *p != '/')
p--;
p++;
this_name = p;
return p;
}
static void __vdie(const char *fmt, va_list ap, int err)
{
int ret = errno;
char *p = get_this_name();
if (err && errno)
perror(p);
else
ret = -1;
fprintf(stderr, " ");
vfprintf(stderr, fmt, ap);
fprintf(stderr, "\n");
exit(ret);
}
void pdie(const char *fmt, ...)
{
va_list ap;
va_start(ap, fmt);
__vdie(fmt, ap, 1);
va_end(ap);
}
static void read_page(struct tep_handle *tep, struct kbuffer *kbuf)
{
static struct trace_seq seq;
struct tep_record record;
if (seq.buffer)
trace_seq_reset(&seq);
else
trace_seq_init(&seq);
while ((record.data = kbuffer_read_event(kbuf, &record.ts))) {
record.size = kbuffer_event_size(kbuf);
kbuffer_next_event(kbuf, NULL);
tep_print_event(tep, &seq, &record,
"%s-%d %9d\t%s: %s", TEP_PRINT_COMM,
TEP_PRINT_PID, TEP_PRINT_TIME, TEP_PRINT_NAME,
TEP_PRINT_INFO);
trace_seq_do_printf(&seq);
trace_seq_reset(&seq);
}
}
static int next_reader_subbuf(int fd, struct trace_buffer_meta *meta, unsigned
long *read)
{
__u32 prev_read, prev_reader, new_reader;
prev_read = READ_ONCE(meta->reader.read);
prev_reader = READ_ONCE(meta->reader.id);
if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0)
pdie("ioctl");
new_reader = READ_ONCE(meta->reader.id);
if (prev_reader != new_reader) {
*read = 0;
printf("NEW READER PAGE: %d\n", new_reader);
} else
*read = prev_read;
return new_reader;
}
static void signal_handler(int unused)
{
exit_requested = true;
}
int main(int argc, char **argv)
{
int page_size, meta_len, data_len, subbuf, fd;
struct trace_buffer_meta *map;
struct tep_handle *tep;
struct kbuffer *kbuf;
unsigned long read;
void *meta, *data;
char path[32];
int cpu;
if (argc != 2)
return -EINVAL;
argv0 = argv[0];
cpu = atoi(argv[1]);
snprintf(path, 32, "per_cpu/cpu%d/trace_pipe_raw", cpu);
tep = tracefs_local_events(NULL);
kbuf = tep_kbuffer(tep);
page_size = getpagesize();
fd = tracefs_instance_file_open(NULL, path, O_RDONLY);
if (fd < 0)
pdie("raw");
meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
if (meta == MAP_FAILED)
pdie("mmap");
map = (struct trace_buffer_meta *)meta;
meta_len = map->meta_page_size;
printf("entries: %lu\n", map->entries);
printf("overrun: %lu\n", map->overrun);
printf("read: %lu\n", map->read);
printf("bpages_touched: %lu\n", map->bpages_touched);
printf("bpages_lost: %lu\n", map->bpages_lost);
printf("bpages_read: %lu\n", map->bpages_read);
printf("bpage_size: %u\n", map->bpage_size);
printf("nr_bpages: %u\n", map->nr_bpages);
data_len = map->bpage_size * map->nr_bpages;
data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, meta_len);
if (data == MAP_FAILED)
pdie("mmap data");
signal(SIGINT, signal_handler);
while (!exit_requested) {
subbuf = next_reader_subbuf(fd, map, &read);
kbuffer_load_subbuffer(kbuf, data + map->bpage_size * subbuf);
while (kbuf->curr < read)
kbuffer_next_event(kbuf, NULL);
read_page(tep, kbuf);
}
munmap(data, data_len);
munmap(meta, page_size);
close(fd);
return 0;
}
Vincent Donnefort (2):
ring-buffer: Introducing ring-buffer mapping functions
tracing: Allow user-space mapping of the ring-buffer
include/linux/ring_buffer.h | 7 +
include/uapi/linux/trace_mmap.h | 31 +++
kernel/trace/ring_buffer.c | 373 +++++++++++++++++++++++++++++++-
kernel/trace/trace.c | 79 ++++++-
4 files changed, 486 insertions(+), 4 deletions(-)
create mode 100644 include/uapi/linux/trace_mmap.h
--
2.43.0.472.g3155946c3a-goog
