[PATCH v3 2/2]: perf record: enable asynchronous trace writing

Alexey Budankov
2018-08-27T18:17:04Z

Trace file offset are linearly calculated by perf_mmap__push() code
for the next possible write operation, but file position is updated by
the kernel only in the second lseek() syscall after the loop.
The first lseek() syscall reads that file position for
the next loop iterations.

record__mmap_read_sync implements sort of a barrier between spilling
ready profiling data to disk.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
---
Changes in v3:
- written comments about nanosleep(0.5ms) call prior aio_suspend()
to cope with intrusiveness of its implementation in glibc;
- written comments about rationale behind coping profiling data
into mmap->data buffer;
---
tools/perf/builtin-record.c | 125 +++++++++++++++++++++++++++++++++++++++++---
tools/perf/util/mmap.c | 36 ++++++++-----
tools/perf/util/mmap.h | 2 +-
3 files changed, 143 insertions(+), 20 deletions(-)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 22ebeb92ac51..4ac61399a09a 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -53,6 +53,7 @@
#include <sys/mman.h>
#include <sys/wait.h>
#include <linux/time64.h>
+#include <aio.h>

struct switch_output {
bool enabled;
@@ -121,6 +122,23 @@ static int record__write(struct record *rec, void *bf, size_t size)
return 0;
}

+static int record__aio_write(int trace_fd, struct aiocb *cblock,
+ void *buf, size_t size, off_t off)
+{
+ cblock->aio_fildes = trace_fd;
+ cblock->aio_buf = buf;
+ cblock->aio_nbytes = size;
+ cblock->aio_offset = off;
+ cblock->aio_sigevent.sigev_notify = SIGEV_NONE;
+
+ if (aio_write(cblock) == -1) {
+ pr_err("failed to queue perf data, error: %m\n");
+ return -1;
+ }
+
+ return 0;
+}
+
static int process_synthesized_event(struct perf_tool *tool,
union perf_event *event,
struct perf_sample *sample __maybe_unused,
@@ -130,12 +148,14 @@ static int process_synthesized_event(struct perf_tool *tool,
return record__write(rec, event, event->header.size);
}

-static int record__pushfn(void *to, void *bf, size_t size)
+static int record__pushfn(void *to, void *bf, size_t size, off_t off)
{
struct record *rec = to;
+ struct perf_mmap *map = bf;

rec->samples++;
- return record__write(rec, bf, size);
+ return record__aio_write(rec->session->data->file.fd, &map->cblock,
+ map->data, size, off);
}

static volatile int done;
@@ -510,13 +530,97 @@ static struct perf_event_header finished_round_event = {
.type = PERF_RECORD_FINISHED_ROUND,
};

+static int record__mmap_read_sync(int trace_fd, struct aiocb **cblocks,
+ int cblocks_size, struct record *rec)
+{
+ size_t rem;
+ ssize_t size;
+ off_t rem_off;
+ int i, aio_ret, aio_errno, do_suspend;
+ struct perf_mmap *md;
+ struct timespec timeout0 = { 0, 0 };
+ struct timespec timeoutS = { 0, 1000 * 500 * 1 }; // 0.5ms
+
+ if (!cblocks_size)
+ return 0;
+
+ do {
+ do_suspend = 0;
+ /* aio_suspend() implementation inside glibc (as of v2.27) is
+ * intrusive and not just blocks waiting io requests completion
+ * but polls requests queue inducing context switches in perf
+ * tool process. When profiling in system wide mode with tracing
+ * context switches the trace may be polluted by context switches
+ * from the perf process and the trace size becomes about 3-5
+ * times bigger than that of when writing the trace serially.
+ * To limit the volume of context switches from perf tool
+ * process nanosleep() call below is added prior aio_suspend()
+ * calling till every half of the kernel timer tick which is
+ * usually 1ms (depends on CONFIG_HZ value).
+ */
+ nanosleep(&timeoutS, NULL);
+ if (aio_suspend((const struct aiocb**)cblocks, cblocks_size, &timeout0)) {
+ if (errno == EAGAIN || errno == EINTR) {
+ do_suspend = 1;
+ continue;
+ } else {
+ pr_err("failed to sync perf data, error: %m\n");
+ break;
+ }
+ }
+ for (i = 0; i < cblocks_size; i++) {
+ if (cblocks[i] == NULL) {
+ continue;
+ }
+ aio_errno = aio_error(cblocks[i]);
+ if (aio_errno == EINPROGRESS) {
+ do_suspend = 1;
+ continue;
+ }
+ size = aio_ret = aio_return((struct aiocb*)cblocks[i]);
+ if (aio_ret < 0) {
+ if (aio_errno == EINTR) {
+ size = 0;
+ } else {
+ pr_err("failed to write perf data, error: %m\n");
+ cblocks[i] = NULL;
+ continue;
+ }
+ }
+ rec->bytes_written += size;
+ md = (struct perf_mmap*)((char*)cblocks[i] -
+ offsetof(struct perf_mmap, cblock));
+ rem = cblocks[i]->aio_nbytes - (size_t)size;
+ if (rem == 0) {
+ perf_mmap__put(md);
+ cblocks[i] = NULL;
+ if (switch_output_size(rec))
+ trigger_hit(&switch_output_trigger);
+ } else {
+ rem_off = cblocks[i]->aio_offset +
+ cblocks[i]->aio_nbytes - rem;
+ if (!record__aio_write(trace_fd,
+ (struct aiocb *)cblocks[i],
+ md->data + cblocks[i]->aio_nbytes - rem,
+ rem, rem_off))
+ do_suspend = 1;
+ }
+ }
+ } while (do_suspend);
+
+ return 0;
+}
+
static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evlist,
bool overwrite)
{
u64 bytes_written = rec->bytes_written;
- int i;
- int rc = 0;
+ int i, rc = 0;
struct perf_mmap *maps;
+ int trace_fd = rec->session->data->file.fd;
+ struct aiocb **mmap_aio = rec->evlist->mmap_aio;
+ int mmap_aio_size = 0;
+ off_t off;

if (!evlist)
return 0;
@@ -528,14 +632,17 @@ static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evli
if (overwrite && evlist->bkw_mmap_state != BKW_MMAP_DATA_PENDING)
return 0;

+ off = lseek(trace_fd, 0, SEEK_CUR);
+
for (i = 0; i < evlist->nr_mmaps; i++) {
struct auxtrace_mmap *mm = &maps[i].auxtrace_mmap;

if (maps[i].base) {
- if (perf_mmap__push(&maps[i], rec, record__pushfn) != 0) {
- rc = -1;
+ rc = perf_mmap__push(&maps[i], rec, record__pushfn, &off);
+ if (rc < 0)
goto out;
- }
+ else if (rc > 0)
+ mmap_aio[mmap_aio_size++] = &maps[i].cblock;
}

if (mm->base && !rec->opts.auxtrace_snapshot_mode &&
@@ -545,6 +652,10 @@ static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evli
}
}

+ record__mmap_read_sync(trace_fd, mmap_aio, mmap_aio_size, rec);
+
+ lseek(trace_fd, off, SEEK_SET);
+
/*
* Mark the round finished in case we wrote
* at least one event.
diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
index e71d46cb01cc..d6eb1f17bead 100644
--- a/tools/perf/util/mmap.c
+++ b/tools/perf/util/mmap.c
@@ -292,11 +292,11 @@ int perf_mmap__read_init(struct perf_mmap *map)
}

int perf_mmap__push(struct perf_mmap *md, void *to,
- int push(void *to, void *buf, size_t size))
+ int push(void *to, void *buf, size_t size, off_t), off_t *off)
{
u64 head = perf_mmap__read_head(md);
unsigned char *data = md->base + page_size;
- unsigned long size;
+ unsigned long size, size0 = 0;
void *buf;
int rc = 0;

@@ -306,25 +306,37 @@ int perf_mmap__push(struct perf_mmap *md, void *to,

size = md->end - md->start;

+ /* Data is copied into the md->data buffer to release space in
+ * the kernel buffer as fast as possible, thru perf_mmap__consume().
+ * That lets the kernel to proceed with storing more profiling
+ * data into the kernel buffer earlier than other per-cpu kernel
+ * buffers are handled.
+ *
+ * Coping can be done in two steps in case chunk of the profiling
+ * data split by upper bound in the kernel buffer. In this case
+ * we first move upper part of data from md->start till the
+ * upper bound and then the reminder from the beginning of
+ * the kernel buffer till the end of the data chunk.
+ */
+
if ((md->start & md->mask) + size != (md->end & md->mask)) {
buf = &data[md->start & md->mask];
- size = md->mask + 1 - (md->start & md->mask);
- md->start += size;
-
- if (push(to, buf, size) < 0) {
- rc = -1;
- goto out;
- }
+ size0 = md->mask + 1 - (md->start & md->mask);
+ md->start += size0;
+ memcpy(md->data, buf, size0);
}

buf = &data[md->start & md->mask];
size = md->end - md->start;
md->start += size;
+ memcpy(md->data + size0, buf, size);

- if (push(to, buf, size) < 0) {
- rc = -1;
+ rc = push(to, md, size0 + size, *off) < 0 ? -1 : 1;
+ if (rc == -1)
goto out;
- }
+
+ perf_mmap__get(md);
+ *off += size0 + size;

md->prev = head;
perf_mmap__consume(md);
diff --git a/tools/perf/util/mmap.h b/tools/perf/util/mmap.h
index 1974e621e36b..6211a3a0c4c3 100644
--- a/tools/perf/util/mmap.h
+++ b/tools/perf/util/mmap.h
@@ -95,7 +95,7 @@ union perf_event *perf_mmap__read_forward(struct perf_mmap *map);
union perf_event *perf_mmap__read_event(struct perf_mmap *map);

int perf_mmap__push(struct perf_mmap *md, void *to,
- int push(void *to, void *buf, size_t size));
+ int push(void *to, void *buf, size_t size, off_t off), off_t *off);

size_t perf_mmap__mmap_len(struct perf_mmap *map);