Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 9 Mar 2023 14:34:05 +0100 | Subject | Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated |
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On Thu, 9 Mar 2023 at 14:05, Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Mar 06, 2023 at 09:24:18PM +0800, Zhang Qiao wrote: > > Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of > > entity being placed") fix an overflowing bug, but ignore > > a case that se->exec_start is reset after a migration. > > > > For fixing this case, we reset the vruntime of a long > > sleeping task in migrate_task_rq_fair(). > > > > > @@ -7635,7 +7653,23 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > > if (READ_ONCE(p->__state) == TASK_WAKING) { > > struct cfs_rq *cfs_rq = cfs_rq_of(se); > > > > - se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > > + /* > > + * We determine whether a task sleeps for long by checking > > + * se->exec_start, and if it is, we sanitize its vruntime at > > + * place_entity(). However, after a migration, this detection > > + * method fails due to se->exec_start being reset. > > + * > > + * For fixing this case, we add the same check here. For a task > > + * which has slept for a long time, its vruntime should be reset > > + * to cfs_rq->min_vruntime with a sleep credit. Because waking > > + * task's vruntime will be added to cfs_rq->min_vruntime when > > + * enqueue, we only need to reset the se->vruntime of waking task > > + * to a credit here. > > + */ > > + if (entity_is_long_sleep(se)) > > + se->vruntime = -sched_sleeper_credit(se); > > + else > > + se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > > } > > *groan*, that again... > > Can't we simply do: > > https://lkml.kernel.org/r/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@linux.intel.com
I'm not sure this would help here, exec_start == 0 can be easily managed in place_entity
The problem stands for task about to be migrated out of taking prev_cpu lock
Then, even if we don't clear exec_start before migrating and keep current value to be used in place_entity on the new cpu, we can't compare the rq_clock_task(rq_of(cfs_rq)) of 2 different rqs AFAICT
side note, why do we reset exec_start ? Is it only to make task_hot returns false ?
> > ?
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