[v2,0/2] Introducing trace buffer mapping by user-space

Message

Vincent Donnefort March 22, 2023, 10:22 a.m. UTC

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 access to userspace to the ring-buffer pages
directly via a mapping. A piece of software can now become a reader of the
ring-buffer, and drive a consuming or non-consuming read in a similar fashion to
what trace and trace_pipe offer.

Attached to this cover letter an example of consuming read for a ring-buffer,
using libtracefs.

Vincent

v1 -> v2:
  * Hide data_pages from the userspace struct
  * Fix META_PAGE_MAX_PAGES
  * Support for order > 0 meta-page
  * Add missing page->mapping.

Vincent Donnefort (2):
  ring-buffer: Introducing ring-buffer mapping functions
  tracing: Allow user-space mapping of the ring-buffer

--

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <signal.h>
#include <errno.h>
#include <unistd.h>
#include <tracefs.h>
#include <kbuffer.h>
#include <event-parse.h>

#include <asm/types.h>
#include <sys/mman.h>
#include <sys/ioctl.h>

#define TRACE_MMAP_IOCTL_GET_READER_PAGE	_IO('T', 0x1)

struct ring_buffer_meta_page_header {
        __u64   entries;
        __u64   overrun;
        __u32   pages_touched;
        __u32   meta_page_size;
        __u32   reader_page;    /* ID of the current reader page */
        __u32   nr_data_pages;  /* doesn't take into account the reader_page */
        __u32   data_page_head; /* ring-buffer head as an offset from data_start */
        __u32   data_start;     /* offset within the meta page */
};

/* 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);
};

static char *argv0;
static bool need_exit;

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,
		      void *data, int page)
{
	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))) {
		kbuffer_next_event(kbuf, NULL);
		tep_print_event(tep, &seq, &record,
				"%s-%d %9d\t%s\n", TEP_PRINT_COMM,
				TEP_PRINT_PID, TEP_PRINT_TIME, TEP_PRINT_NAME);
		trace_seq_do_printf(&seq);
		trace_seq_reset(&seq);
	}
}

static int next_reader_page(int fd, struct ring_buffer_meta_page_header *meta,
			    struct kbuffer *kbuf)
{
	int prev_reader_page = meta->reader_page;

	if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER_PAGE) < 0)
		pdie("ioctl");

	return meta->reader_page;
}

static void signal_handler(int unused)
{
	printf("Exit!\n");
	need_exit = true;
}

int main(int argc, char **argv)
{
	int page_size, meta_len, data_len, page, fd, start = -1;
	struct ring_buffer_meta_page_header *map;
	struct kbuffer *kbuf, prev_kbuf;
	struct tep_handle *tep;
	__u64 prev_entries;
	void *meta, *data;
	char *buf, path[32];
	int cpu;

	argv0 = argv[0];
	cpu = atoi(argv[1]);
	snprintf(path, 32, "per_cpu/cpu%d/trace_pipe_raw", cpu);

	signal(SIGINT, signal_handler);
	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 = meta;
	meta_len = map->meta_page_size;

        if (meta_len > page_size) {
                munmap(meta, page_size);
                meta = mmap(NULL, meta_len, PROT_READ, MAP_SHARED, fd, 0);
                if (meta == MAP_FAILED)
                        pdie("mmap");
                map = meta;
        }

	printf("entries:	%llu\n", map->entries);
	printf("overrun:	%llu\n", map->overrun);
	printf("pages_touched:	%u\n", map->pages_touched);
	printf("reader_page:	%u\n", map->reader_page);
	printf("nr_data_pages:	%u\n\n", map->nr_data_pages);

	data_len = page_size * (map->nr_data_pages + 1);

	data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, meta_len);
	if (data == MAP_FAILED)
		pdie("mmap data");

	page = ((struct ring_buffer_meta_page_header *)meta)->reader_page;
again:
	do {
		kbuffer_load_subbuffer(kbuf, data + page_size * page);

		if (page != start) {
			printf("READER PAGE: %d\n", map->reader_page);
		} else {
			kbuf->curr = prev_kbuf.curr;
			kbuf->index = prev_kbuf.index;
			kbuf->next = prev_kbuf.next;
			kbuf->timestamp = prev_kbuf.timestamp;
			kbuffer_next_event(kbuf, NULL);
		}

		prev_entries = map->entries;
		start = page;

		read_page(tep, kbuf, data, page);
	} while ((page = next_reader_page(fd, meta, kbuf)) != start);

	prev_kbuf.curr = kbuf->curr;
	prev_kbuf.index = kbuf->index;
	prev_kbuf.next = kbuf->next;
	prev_kbuf.timestamp = kbuf->timestamp;

	while (prev_entries == *(volatile __u64 *)&map->entries && !need_exit)
		usleep(100000);

	if (!need_exit)
		goto again;

	munmap(data, data_len);
	munmap(meta, page_size);
	close(fd);

	return 0;
}


 include/linux/ring_buffer.h     |   8 +
 include/uapi/linux/trace_mmap.h |  28 +++
 kernel/trace/ring_buffer.c      | 384 +++++++++++++++++++++++++++++++-
 kernel/trace/trace.c            |  76 ++++++-
 4 files changed, 490 insertions(+), 6 deletions(-)
 create mode 100644 include/uapi/linux/trace_mmap.h