linux-kernel - [PATCH rcu/urgent 1/2] scftorture: Avoid false-positive warnings in scftorture

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Date:   Wed, 14 Jul 2021 13:50:42 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        valentin.schneider@....com, "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu/urgent 1/2] scftorture: Avoid false-positive warnings in scftorture_invoker()

If the call to set_cpus_allowed_ptr() in scftorture_invoker()
fails, a later WARN_ONCE() complains.  But with the advent of
570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of
nr_cpus_allowed"), this complaint can be drowned out by complaints from
smp_processor_id().  The rationale for this change is that scftorture's
kthreads are not marked with PF_NO_SETAFFINITY, which means that a system
administrator could change affinity at any time.

However, scftorture is a torture test, and the system administrator might
well have a valid test-the-test reason for changing affinity.  This commit
therefore changes to raw_smp_processor_id() in order to avoid the noise,
and also adds a WARN_ON_ONCE() to the call to set_cpus_allowed_ptr() in
order to directly detect immediate failure.  There is no WARN_ON_ONCE()
within the test loop, allowing human-reflex-based affinity resetting,
if desired.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/scftorture.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/scftorture.c b/kernel/scftorture.c
index 2377cbb32474..29e8fc5d91a7 100644
--- a/kernel/scftorture.c
+++ b/kernel/scftorture.c
@@ -405,15 +405,15 @@ static int scftorture_invoker(void *arg)

 	VERBOSE_SCFTORTOUT("scftorture_invoker %d: task started", scfp->cpu);
 	cpu = scfp->cpu % nr_cpu_ids;
-	set_cpus_allowed_ptr(current, cpumask_of(cpu));
+	WARN_ON_ONCE(set_cpus_allowed_ptr(current, cpumask_of(cpu)));
 	set_user_nice(current, MAX_NICE);
 	if (holdoff)
 		schedule_timeout_interruptible(holdoff * HZ);

-	VERBOSE_SCFTORTOUT("scftorture_invoker %d: Waiting for all SCF torturers from cpu %d", scfp->cpu, smp_processor_id());
+	VERBOSE_SCFTORTOUT("scftorture_invoker %d: Waiting for all SCF torturers from cpu %d", scfp->cpu, raw_smp_processor_id());

 	// Make sure that the CPU is affinitized appropriately during testing.
-	curcpu = smp_processor_id();
+	curcpu = raw_smp_processor_id();
 	WARN_ONCE(curcpu != scfp->cpu % nr_cpu_ids,
 		  "%s: Wanted CPU %d, running on %d, nr_cpu_ids = %d\n",
 		  __func__, scfp->cpu, curcpu, nr_cpu_ids);
-- 
2.31.1.189.g2e36527f23