[PATCH 5/7] perf bench futex: Fix hung wakeup tasks after requeueing

From: Arnaldo Carvalho de Melo
Date: Mon Apr 27 2015 - 13:05:22 EST


From: Davidlohr Bueso <dave@xxxxxxxxxxxx>

The futex-requeue benchmark can hang because of missing wakeups once the
benchmark is done, ie:

[Run 1]: Requeued 1024 of 1024 threads in 0.3290 ms
perf: couldn't wakeup all tasks (135/1024)

This bug, while perhaps suggesting missing wakeups in kernel futex code,
is merely a consequence of the crappy FUTEX_CMP_REQUEUE man page,
incorrectly mentioning that the number of requeued tasks is in fact
returned, not the wakeups.

This patch acknowledges this and updates the corresponding futex_wake
code around it.

Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Link: http://lkml.kernel.org/r/1429894848.10273.44.camel@xxxxxxxxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
tools/perf/bench/futex-requeue.c | 15 ++++++++-------
1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/tools/perf/bench/futex-requeue.c b/tools/perf/bench/futex-requeue.c
index bedff6b5b3cf..ad0d9b5342fb 100644
--- a/tools/perf/bench/futex-requeue.c
+++ b/tools/perf/bench/futex-requeue.c
@@ -132,6 +132,9 @@ int bench_futex_requeue(int argc, const char **argv,
if (!fshared)
futex_flag = FUTEX_PRIVATE_FLAG;

+ if (nrequeue > nthreads)
+ nrequeue = nthreads;
+
printf("Run summary [PID %d]: Requeuing %d threads (from [%s] %p to %p), "
"%d at a time.\n\n", getpid(), nthreads,
fshared ? "shared":"private", &futex1, &futex2, nrequeue);
@@ -161,20 +164,18 @@ int bench_futex_requeue(int argc, const char **argv,

/* Ok, all threads are patiently blocked, start requeueing */
gettimeofday(&start, NULL);
- for (nrequeued = 0; nrequeued < nthreads; nrequeued += nrequeue) {
+ while (nrequeued < nthreads) {
/*
* Do not wakeup any tasks blocked on futex1, allowing
* us to really measure futex_wait functionality.
*/
- futex_cmp_requeue(&futex1, 0, &futex2, 0,
- nrequeue, futex_flag);
+ nrequeued += futex_cmp_requeue(&futex1, 0, &futex2, 0,
+ nrequeue, futex_flag);
}
+
gettimeofday(&end, NULL);
timersub(&end, &start, &runtime);

- if (nrequeued > nthreads)
- nrequeued = nthreads;
-
update_stats(&requeued_stats, nrequeued);
update_stats(&requeuetime_stats, runtime.tv_usec);

@@ -184,7 +185,7 @@ int bench_futex_requeue(int argc, const char **argv,
}

/* everybody should be blocked on futex2, wake'em up */
- nrequeued = futex_wake(&futex2, nthreads, futex_flag);
+ nrequeued = futex_wake(&futex2, nrequeued, futex_flag);
if (nthreads != nrequeued)
warnx("couldn't wakeup all tasks (%d/%d)", nrequeued, nthreads);

--
1.9.3

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/