On Tue, Jul 09, 2019 at 12:57:59PM +0100, Chris Redpath wrote: > The ancient workaround to avoid the cost of updating rq clocks in the > middle of a migration causes some issues on asymmetric CPU capacity > systems where we use task utilization to determine which cpus fit a task. > On quiet systems we can inflate task util after a migration which > causes misfit to fire and force-migrate the task. > > This occurs when: > > (a) a task has util close to the non-overutilized capacity limit of a > particular cpu (cpu0 here); and > (b) the prev_cpu was quiet otherwise, such that rq clock is > sufficiently out of date (cpu1 here). > > e.g. > _____ > cpu0: ________________________| |______________ > > |<- misfit happens > ______ ___ ___ > cpu1: ____| |______________|___| |_________| > > ->| |<- wakeup migration time > last rq clock update > > When the task util is in just the right range for the system, we can end > up migrating an unlucky task back and forth many times until we are lucky > and the source rq happens to be updated close to the migration time. > > In order to address this, lets update both rq_clock and cfs_rq where > this could be an issue.
Can you quantify how much of a problem this really is? It is really sad, but this is already the second place where we take rq->lock on migration. We worked so hard to avoid having to acquire it :/ > Signed-off-by: Chris Redpath <[email protected]> > --- > kernel/sched/fair.c | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index b798fe7ff7cd..51791db26a2a 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6545,6 +6545,21 @@ static void migrate_task_rq_fair(struct task_struct > *p, int new_cpu) > * wakee task is less decayed, but giving the wakee more load > * sounds not bad. > */ > + if (static_branch_unlikely(&sched_asym_cpucapacity) && > + p->state == TASK_WAKING) { nit: indent fail. > + /* > + * On asymmetric capacity systems task util guides > + * wake placement so update rq_clock and cfs_rq > + */ > + struct cfs_rq *cfs_rq = task_cfs_rq(p); > + struct rq *rq = task_rq(p); > + struct rq_flags rf; > + > + rq_lock_irqsave(rq, &rf); > + update_rq_clock(rq); > + update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), > cfs_rq); > + rq_unlock_irqrestore(rq, &rf); > + } > remove_entity_load_avg(&p->se); > } > > -- > 2.17.1 >
