Messages in this thread Patch in this message | | | Date | Thu, 18 Feb 2021 06:43:06 +0530 | From | Pavan Kondeti <> | Subject | Re: [PATCH] sched/fair: Ignore percpu threads for imbalance pulls |
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On Wed, Feb 17, 2021 at 02:50:23PM +0000, Valentin Schneider wrote: > On 17/02/21 17:38, Lingutla Chandrasekhar wrote: > > In load balancing, when balancing group is unable to pull task > > due to ->cpus_ptr constraints from busy group, then it sets > > LBF_SOME_PINNED to lb env flags, as a consequence, sgc->imbalance > > is set for its parent domain level. which makes the group > > classified as imbalance to get help from another balancing cpu. > > > > Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and > > CPUs 2-3 as Bigs with below scenario: > > - CPU0 doing newly_idle balancing > > - CPU1 running percpu kworker and RT task (small tasks) > > - CPU2 running 2 big tasks > > - CPU3 running 1 medium task > > > > While CPU0 is doing newly_idle load balance at MC level, it fails to > > pull percpu kworker from CPU1 and sets LBF_SOME_PINNED to lb env flag > > and set sgc->imbalance at DIE level domain. As LBF_ALL_PINNED not cleared, > > it tries to redo the balancing by clearing CPU1 in env cpus, but it don't > > find other busiest_group, so CPU0 stops balacing at MC level without > > clearing 'sgc->imbalance' and restart the load balacing at DIE level. > > > > And CPU0 (balancing cpu) finds LITTLE's group as busiest_group with group > > type as imbalance, and Bigs that classified the level below imbalance type > > would be ignored to pick as busiest, and the balancing would be aborted > > without pulling any tasks (by the time, CPU1 might not have running tasks). > > > > It is suboptimal decision to classify the group as imbalance due to > > percpu threads. So don't use LBF_SOME_PINNED for per cpu threads. > > > > Sounds like you've stumbled on the same thing I'm trying to fix in > > http://lore.kernel.org/r/20210128183141.28097-8-valentin.schneider@arm.com > > (I'm currently working on a v2) > > Now, I'd tend to agree that if we could prevent pcpu kworkers from > interfering with load-balance altogether, that would indeed be much > better than trying to deal with the group_imbalanced faff further down the > line (which is what I've been doing). > > > Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org> > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 04a3ce20da67..44a05ad8c96b 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -7560,7 +7560,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) > > > > schedstat_inc(p->se.statistics.nr_failed_migrations_affine); > > > > - env->flags |= LBF_SOME_PINNED; > > + /* Ignore percpu threads for imbalance pulls. */ > > + if (p->nr_cpus_allowed > 1) > > + env->flags |= LBF_SOME_PINNED; > > > > /* > > * Remember if this task can be migrated to any other CPU in > > Unlike user tasks, pcpu kworkers have a stable affinity (with some hotplug > quirks), so perhaps we could do this instead: > > --- > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 8a8bd7b13634..84fca350b9ae 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7539,6 +7539,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) > if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu)) > return 0; > > + if (kthread_is_per_cpu(p)) > + return 0; > + > if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) { > int cpu; >
Looks good to me. In our testing also, the false imbalance is manifested due to pinned kworkers.
Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
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