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Date: Tue, 25 Aug 2020 08:29:35 +0530 From: Srikar Dronamraju <srikar@...ux.vnet.ibm.com> To: xunlei <xlpang@...ux.alibaba.com> Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Vincent Guittot <vincent.guittot@...aro.org>, Juri Lelli <juri.lelli@...hat.com>, Wetp Zhang <wetp.zy@...ux.alibaba.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH] sched/fair: Fix wrong cpu selecting from isolated domain * xunlei <xlpang@...ux.alibaba.com> [2020-08-25 10:11:24]: > On 2020/8/24 PM9:38, Srikar Dronamraju wrote: > > * Xunlei Pang <xlpang@...ux.alibaba.com> [2020-08-24 20:30:19]: > > > >> We've met problems that occasionally tasks with full cpumask > >> (e.g. by putting it into a cpuset or setting to full affinity) > >> were migrated to our isolated cpus in production environment. > >> > >> After some analysis, we found that it is due to the current > >> select_idle_smt() not considering the sched_domain mask. > >> > >> Fix it by checking the valid domain mask in select_idle_smt(). > >> > >> Fixes: 10e2f1acd010 ("sched/core: Rewrite and improve select_idle_siblings()) > >> Reported-by: Wetp Zhang <wetp.zy@...ux.alibaba.com> > >> Signed-off-by: Xunlei Pang <xlpang@...ux.alibaba.com> > >> --- > >> kernel/sched/fair.c | 9 +++++---- > >> 1 file changed, 5 insertions(+), 4 deletions(-) > >> > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> index 1a68a05..fa942c4 100644 > >> --- a/kernel/sched/fair.c > >> +++ b/kernel/sched/fair.c > >> @@ -6075,7 +6075,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int > >> /* > >> * Scan the local SMT mask for idle CPUs. > >> */ > >> -static int select_idle_smt(struct task_struct *p, int target) > >> +static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target) > >> { > >> int cpu; > >> > >> @@ -6083,7 +6083,8 @@ static int select_idle_smt(struct task_struct *p, int target) > >> return -1; > >> > >> for_each_cpu(cpu, cpu_smt_mask(target)) { > >> - if (!cpumask_test_cpu(cpu, p->cpus_ptr)) > >> + if (!cpumask_test_cpu(cpu, p->cpus_ptr) || > >> + !cpumask_test_cpu(cpu, sched_domain_span(sd))) > >> continue; > > > > Don't think this is right thing to do. What if this task had set a cpumask > > that doesn't cover all the cpus in this sched_domain_span(sd) ah, right I missed the 'or' part. > > It doesn't matter, without this patch, it selects an idle cpu from: > "cpu_smt_mask(target) and p->cpus_ptr" > > with this patch, it selects an idle cpu from: > "cpu_smt_mask(target) and p->cpus_ptr and sched_domain_span(sd)" > > > > > cpu_smt_mask(target) would already limit to the sched_domain_span(sd) so I > > am not sure how this can help? > > > > > > Here is an example: > CPU0 and CPU16 are hyper-thread pair, CPU16 is domain isolated. So its > sd_llc doesn't contain CPU16, and cpu_smt_mask(0) is 0 and 16. > > Then we have @target is 0, select_idle_smt() may return the isolated(and > idle) CPU16 without this patch. Okay. -- Thanks and Regards Srikar Dronamraju
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