linux-kernel - Re: [PATCH v5 2/5] sched/fair: Limited scan for idle cores when overloaded

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Date:   Thu, 15 Sep 2022 11:08:43 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Josh Don <joshdon@...gle.com>, Chen Yu <yu.c.chen@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v5 2/5] sched/fair: Limited scan for idle cores when
 overloaded

Hi Tim, thanks for your reviewing!

On 9/15/22 6:25 AM, Tim Chen wrote:
> On Fri, 2022-09-09 at 13:53 +0800, Abel Wu wrote:
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 5af9bf246274..7abe188a1533 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6437,26 +6437,42 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>   		time = cpu_clock(this);
>>   	}
>>   
>> -	if (sched_feat(SIS_UTIL) && !has_idle_core) {
>> +	if (sched_feat(SIS_UTIL)) {
>>   		sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
>>   		if (sd_share) {
>>   			/* because !--nr is the condition to stop scan */
>>   			nr = READ_ONCE(sd_share->nr_idle_scan) + 1;
>> -			/* overloaded LLC is unlikely to have idle cpu/core */
>> -			if (nr == 1)
>> +
>> +			/*
>> +			 * Overloaded LLC is unlikely to have idle cpus.
>> +			 * But if has_idle_core hint is true, a limited
>> +			 * speculative scan might help without incurring
>> +			 * much overhead.
>> +			 */
>> +			if (has_idle_core)
>> +				nr = nr > 1 ? INT_MAX : 3;
> 
> The choice of nr is a very abrupt function of utilization when has_idle_core==true,
> it is either feast or famine.  Why is such choice better than a smoother
> reduction of nr vs utilization?  I agree that we want to scan more aggressively than
> !has_idle_core, but it is not obvious why the above work better, versus something
> like nr = nr*2+1.
This has been discussed with Mel, and he suggested do simple things
first before scaling the depth.

https://lore.kernel.org/all/20220906095717.maao4qtel4fhbmfq@techsingularity.net/

Thanks and BR,
Abel