On 29/01/21 01:02, Tao Zhou wrote: > Hi, > > On Wed, Jan 27, 2021 at 07:30:35PM +0000, Valentin Schneider wrote: > >> Fiddling some more with a TLA+ model of set_cpus_allowed_ptr() & friends >> unearthed one more outstanding issue. This doesn't even involve >> migrate_disable(), but rather affinity changes and execution of the stopper >> racing with each other. >> >> My own interpretation of the (lengthy) TLA+ splat (note the potential for >> errors at each level) is: >> >> Initial conditions: >> victim.cpus_mask = {CPU0, CPU1} >> >> CPU0 CPU1 >> CPU<don't care> >> >> switch_to(victim) >> >> set_cpus_allowed(victim, {CPU1}) >> kick CPU0 >> migration_cpu_stop({.dest_cpu = CPU1}) >> switch_to(stopper/0) >> // e.g. CFS >> load balance >> >> move_queued_task(CPU0, victim, CPU1); > ^^^^^^^^^^^^^^^^ > > Why is move_queued_task() not attach_task()/detach_task() for CFS load.. >
Heh I expected that one; it is indeed detach_task()/attach_task() for CFS LB. I didn't want to make this any longer than it needed to, and I figured that move_queued_task() being a composition of detach_task(), attach_task() and rq_locks, this would get the point across. This does raise an "interesting" point that ATM I think this issue cannot actually involve move_queued_task(), since all current move_queued_task() callsites are issued either from a stopper or from set_cpus_allowed_ptr(). CFS' detach_task() + attach_task() could do it, though. >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 06b449942adf..b57326b0a742 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -1923,20 +1923,28 @@ static int migration_cpu_stop(void *data) >> complete = true; >> } >> >> - /* migrate_enable() -- we must not race against SCA */ >> - if (dest_cpu < 0) { >> - /* >> - * When this was migrate_enable() but we no longer >> - * have a @pending, a concurrent SCA 'fixed' things >> - * and we should be valid again. Nothing to do. >> - */ >> - if (!pending) { >> - WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), >> &p->cpus_mask)); >> - goto out; >> - } >> + /* >> + * When this was migrate_enable() but we no longer >> + * have a @pending, a concurrent SCA 'fixed' things >> + * and we should be valid again. >> + * >> + * This can also be a stopper invocation that was 'fixed' by an >> + * earlier one. >> + * >> + * Nothing to do. >> + */ >> + if ((dest_cpu < 0 || dest_cpu == cpu_of(rq)) && !pending) { > > When the condition 'dest_cpu == cpu_of(rq)' is true, pending is not NULL. > The condition may be like this: > > if ((dest_cpu < 0 && !pending) || dest_cpu == cpu_of(rq)) > > We want to choose one cpu in the new(currently modified) cpu_mask and > complete all. > Consider the execution of migration_cpu_stop() in above trace with migrate_task_to(). We do have: - dest_cpu == cpu_of(rq) - p->migration_pending but we do *not* want to bail out at this condition, because we need to fix up dest_cpu.