Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched: Better document ttwu() | Date | Sun, 05 Jul 2020 01:21:57 +0100 |
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On 03/07/20 14:32, Peter Zijlstra wrote: > Dave hit the problem fixed by commit: > > b6e13e85829f ("sched/core: Fix ttwu() race") > > and failed to understand much of the code involved. Per his request a > few comments to (hopefully) clarify things. > > Requested-by: Dave Chinner <david@fromorbit.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Small extra comment below, otherwise FWIW:
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> +++ b/kernel/sched/core.c > @@ -79,6 +79,100 @@ __read_mostly int scheduler_running; > */ > int sysctl_sched_rt_runtime = 950000; > > + > +/* > + * Serialization rules: > + * > + * Lock order: > + * > + * p->pi_lock > + * rq->lock > + * hrtimer_cpu_base->lock (hrtimer_start() for bandwidth controls) > + * > + * rq1->lock > + * rq2->lock where: rq1 < rq2 > + * > + * Regular state: > + * > + * Normal scheduling state is serialized by rq->lock. __schedule() takes the > + * local CPU's rq->lock, it optionally removes the task from the runqueue and > + * always looks at the local rq data structures to find the most elegible task > + * to run next. > + * > + * Task enqueue is also under rq->lock, possibly taken from another CPU. > + * Wakeups from another LLC domain might use an IPI to transfer the enqueue to > + * the local CPU to avoid bouncing the runqueue state around [ see > + * ttwu_queue_wakelist() ] > + * > + * Task wakeup, specifically wakeups that involve migration, are horribly > + * complicated to avoid having to take two rq->locks. > + * > + * Special state: > + * > + * System-calls and anything external will use task_rq_lock() which acquires > + * both p->pi_lock and rq->lock. As a consequence the state they change is > + * stable while holding either lock: > + * > + * - sched_setaffinity()/ > + * set_cpus_allowed_ptr(): p->cpus_ptr, p->nr_cpus_allowed > + * - set_user_nice(): p->se.load, p->*prio > + * - __sched_setscheduler(): p->sched_class, p->policy, p->*prio, > + * p->se.load, p->rt_priority, > + * p->dl.dl_{runtime, deadline, period, flags, bw, density}
Uclamp stuff can also get updated in __sched_setscheduler(); see __setscheduler_uclamp(). It's only p->uclamp_req AFAICT, but I don't think there's harm in just saying p->uclamp*.
> + * - sched_setnuma(): p->numa_preferred_nid > + * - sched_move_task()/ > + * cpu_cgroup_fork(): p->sched_task_group > + * - uclamp_update_active() p->uclamp* > + *
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