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Subject: Re: [PATCH] sched/fair: Optimize select_idle_cpu
To:     Valentin Schneider <valentin.schneider@arm.com>,
        Peter Zijlstra <peterz@infradead.org>
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>
References: <20191212144102.181510-1-cj.chengjian@huawei.com>
 <20191212152406.GB2827@hirez.programming.kicks-ass.net>
 <2498b792-8549-5ffd-0d85-179740e356a0@arm.com>
From:   "chengjian (D)" <cj.chengjian@huawei.com>
Message-ID: <bc0c48aa-4b98-b8e6-f828-762c4b90bfab@huawei.com>
Date:   Fri, 13 Dec 2019 17:47:08 +0800
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On 2019/12/12 23:44, Valentin Schneider wrote:
> On 12/12/2019 15:24, Peter Zijlstra wrote:
>> On Thu, Dec 12, 2019 at 10:41:02PM +0800, Cheng Jian wrote:
>>
>>> Fixes: 1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
>> The 'funny' thing is that select_idle_core() actually does the right
>> thing.
>>
>> Copying that should work:
>>
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 08a233e97a01..416d574dcebf 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5828,6 +5837,7 @@ static inline int select_idle_smt(struct task_struct *p, int target)
>>    */
>>   static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
>>   {
>> +	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
>>   	struct sched_domain *this_sd;
>>   	u64 avg_cost, avg_idle;
>>   	u64 time, cost;
>> @@ -5859,11 +5869,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>>   
>>   	time = cpu_clock(this);
>>   
>> -	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
>> +	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
>> +
>> +	for_each_cpu_wrap(cpu, cpus, target) {
>>   		if (!--nr)
>>   			return si_cpu;
>> -		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>> -			continue;
>>   		if (available_idle_cpu(cpu))
>>   			break;
>>   		if (si_cpu == -1 && sched_idle_cpu(cpu))
>>
> That looks sane enough. I'd only argue the changelog should directly point
> out that the issue is we consume some CPUs out of 'nr' that are not allowed
> for the task and thus waste our attempts.
>
> .


Hi，Valentin

Yeah，Yours are more clear.

Thank you so much.


-- Cheng Jian