Messages in this thread Patch in this message | | | Date | Mon, 22 Feb 2021 11:03:46 +0530 | From | Pavan Kondeti <> | Subject | Re: [PATCH v2 1/7] sched/fair: Ignore percpu threads for imbalance pulls |
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On Fri, Feb 19, 2021 at 12:59:57PM +0000, Valentin Schneider wrote: > From: Lingutla Chandrasekhar <clingutla@codeaurora.org> > > 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. > > Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org> > [Use kthread_is_per_cpu() rather than p->nr_cpus_allowed] > Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> > --- > kernel/sched/fair.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 8a8bd7b13634..2d4dcf1a3372 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7539,6 +7539,10 @@ 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; > > + /* Disregard pcpu kthreads; they are where they need to be. */ > + if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p)) > + return 0; > + > if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) { > int cpu; >
Looks good to me. Thanks Valentin for the help.
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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