On 17/11/20 11:06, Peter Zijlstra wrote: > On Mon, Nov 16, 2020 at 10:00:14AM +0000, Valentin Schneider wrote: >> >> On 15/11/20 22:32, Oleksandr Natalenko wrote: >> > Hi. >> > >> > I'm running v5.10-rc3-rt7 for some time, and I came across this splat in >> > dmesg: >> > >> > ``` >> > [118769.951010] ------------[ cut here ]------------ >> > [118769.951013] WARNING: CPU: 19 PID: 146 at kernel/sched/core.c:2013 >> >> Err, I didn't pick up on this back then, but isn't that check bogus? If the >> task is enqueued elsewhere, it's valid for it not to be affined >> 'here'. Also that is_migration_disabled() check within is_cpu_allowed() >> makes me think this isn't the best thing to call on a remote task. >> >> --- >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 1218f3ce1713..47d5b677585f 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -2010,7 +2010,7 @@ static int migration_cpu_stop(void *data) >> * valid again. Nothing to do. >> */ >> if (!pending) { >> - WARN_ON_ONCE(!is_cpu_allowed(p, cpu_of(rq))); >> + WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), >> p->cpus_ptr)); > > Ho humm.. bit of a mess that. I'm trying to figure out if we need that > is_per_cpu_kthread() test here or not. > > I suppose not, what we want here is to ensure the CPU is in cpus_mask > and not care about the whole hotplug mess. >
That was my thought as well. On top of that, is_cpu_allowed(p) does a p->migration_disabled read, which isn't so great in the remote case. > Would it makes sense to replace both instances in migration_cpu_stop() > with: > > WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), p->cpus_mask)); > > ? I guess so; I was trying to see if we could factorize this, but stopped mid-swing as I'm really wary of shuffling too much of this code (even with the help of TLA+; well, maybe *because* of it).