linux-kernel - [PATCH 3/3] tools/perf: Fix timing issue with parallel threads in perf bench wake-up-parallel

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Date: Fri,  7 Jun 2024 10:13:54 +0530
From: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
To: acme@...nel.org, jolsa@...nel.org, adrian.hunter@...el.com,
        irogers@...gle.com, namhyung@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, akanksha@...ux.ibm.com,
        maddy@...ux.ibm.com, atrajeev@...ux.vnet.ibm.com, kjain@...ux.ibm.com,
        disgoel@...ux.vnet.ibm.com
Subject: [PATCH 3/3] tools/perf: Fix timing issue with parallel threads in perf bench wake-up-parallel

perf bench futex fails as below and hangs intermittently when
attempted to run on on a powerpc system:

./perf bench futex wake-parallel
 Running 'futex/wake-parallel' benchmark:
 Run summary [PID 88588]: blocking on 640 threads (at [private] futex 0x10464b8c), 640 threads waking up 1 at a time.

[Run 1]: Avg per-thread latency (waking 1/640 threads) in 0.1309 ms (+-53.27%)
[Run 2]: Avg per-thread latency (waking 1/640 threads) in 0.0120 ms (+-31.16%)
[Run 3]: Avg per-thread latency (waking 1/640 threads) in 0.1474 ms (+-92.47%)
[Run 4]: Avg per-thread latency (waking 1/640 threads) in 0.2883 ms (+-67.75%)
[Run 5]: Avg per-thread latency (waking 1/640 threads) in 0.4108 ms (+-39.60%)
[Run 6]: Avg per-thread latency (waking 1/640 threads) in 0.7843 ms (+-78.98%)
perf: couldn't wakeup all tasks (0/1)
perf: couldn't wakeup all tasks (0/1)
perf: couldn't wakeup all tasks (0/1)
perf: couldn't wakeup all tasks (0/1)
perf: couldn't wakeup all tasks (0/1)
perf: couldn't wakeup all tasks (0/1)

In the system, where perf bench wake-up-parallel is has system
configuration of 640 cpus. After debugging, this turned out to be
a timing issue. The benchmark creates threads equal to number of
cpus and issues a futex_wait. Then it does a usleep for .1 second
before initiating futex_wake. In system configuration with more
threads, the usleep time is not enough. Patch changes the usleep
from 100000 to 200000

With the patch, ran multiple iterations and there were no issues
further seen

Reported-by: Disha Goel <disgoel@...ux.vnet.ibm.com>
Signed-off-by: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
---
 tools/perf/bench/futex-wake-parallel.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/bench/futex-wake-parallel.c b/tools/perf/bench/futex-wake-parallel.c
index 90a5b91bf139..4352e318631e 100644
--- a/tools/perf/bench/futex-wake-parallel.c
+++ b/tools/perf/bench/futex-wake-parallel.c
@@ -318,7 +318,7 @@ int bench_futex_wake_parallel(int argc, const char **argv)
 		cond_broadcast(&thread_worker);
 		mutex_unlock(&thread_lock);
 
-		usleep(100000);
+		usleep(200000);
 
 		/* Ok, all threads are patiently blocked, start waking folks up */
 		wakeup_threads(waking_worker);
-- 
2.43.0