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Date:   Mon, 24 Aug 2020 19:08:20 +0530
From:   Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To:     Xunlei Pang <xlpang@linux.alibaba.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Wetp Zhang <wetp.zy@linux.alibaba.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/fair: Fix wrong cpu selecting from isolated domain
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Reply-To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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* Xunlei Pang <xlpang@linux.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@linux.alibaba.com>
> Signed-off-by: Xunlei Pang <xlpang@linux.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)

cpu_smt_mask(target) would already limit to the sched_domain_span(sd) so I
am not sure how this can help?


-- 
Thanks and Regards
Srikar Dronamraju