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Date:   Thu, 12 Dec 2019 16:04:29 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Cheng Jian <cj.chengjian@huawei.com>
Cc:     mingo@kernel.org, linux-kernel@vger.kernel.org,
        chenwandun@huawei.com, xiexiuqi@huawei.com, liwei391@huawei.com,
        huawei.libin@huawei.com, bobo.shaobowang@huawei.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org
Subject: Re: [PATCH] sched/fair: Optimize select_idle_cpu
Message-ID: <20191212150429.GZ2827@hirez.programming.kicks-ass.net>
References: <20191212144102.181510-1-cj.chengjian@huawei.com>
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On Thu, Dec 12, 2019 at 10:41:02PM +0800, Cheng Jian wrote:
> select_idle_cpu will scan the LLC domain for idle CPUs,
> it's always expensive. so commit
>     1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
> introduces a way to limit how many CPUs we scan.
> 
> But this also lead to the following issue:
> 
> Our threads are all bind to the front CPUs of the LLC domain,
> and now all the threads runs on the last CPU of them. nr is
> always less than the cpumask_weight, for_each_cpu_wrap can't
> find the CPU which our threads can run on, so the threads stay
> at the last CPU all the time.
> 
> Fixes: 1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
> Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
> ---
>  kernel/sched/fair.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..16a29b570803 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5834,6 +5834,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	s64 delta;
>  	int this = smp_processor_id();
>  	int cpu, nr = INT_MAX, si_cpu = -1;
> +	struct cpumask cpus;

NAK, you must not put a cpumask on stack.