linux-kernel - Re: [PATCH 1/5] sched/fair: ignore SIS

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Message-ID: <4dde05be-8470-5984-0a30-ba077b9fe6bd@bytedance.com>
Date:   Thu, 14 Jul 2022 14:58:59 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Yicong Yang <yangyicong@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     yangyicong@...ilicon.com, Josh Don <joshdon@...gle.com>,
        Chen Yu <yu.c.chen@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core


On 7/14/22 2:19 PM, Yicong Yang Wrote:
> On 2022/7/12 16:20, Abel Wu wrote:
>> When SIS_UTIL is enabled, SIS domain scan will be skipped if
>> the LLC is overloaded. Since the overloaded status is checked
>> in the load balancing at LLC level, the interval is llc_size
>> miliseconds. The duration might be long enough to affect the
>> overall system throughput if idle cores are out of reach in
>> SIS domain scan.
>>
>> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
>> ---
>>   kernel/sched/fair.c | 15 +++++++++------
>>   1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index a78d2e3b9d49..cd758b3616bd 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6392,16 +6392,19 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>   	struct sched_domain *this_sd;
>>   	u64 time = 0;
>>   
>> -	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>> -	if (!this_sd)
>> -		return -1;
>> -
>>   	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
>>   
>> -	if (sched_feat(SIS_PROP) && !has_idle_core) {
>> +	if (has_idle_core)
>> +		goto scan;
>> +
>> +	if (sched_feat(SIS_PROP)) {
>>   		u64 avg_cost, avg_idle, span_avg;
>>   		unsigned long now = jiffies;
>>   
>> +		this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>> +		if (!this_sd)
>> +			return -1;
>> +
> 
> I don't follow the change here. True that this_sd is used only in SIS_PROP, but it seems irrelevant with your
> commit. Does the position of this make any performance difference?

No, this change doesn't make much difference to performance. Are
you suggesting that I should make this a separate patch?

Thanks,
Abel

> 
> Thanks.
> 
>>   		/*
>>   		 * If we're busy, the assumption that the last idle period
>>   		 * predicts the future is flawed; age away the remaining
>> @@ -6436,7 +6439,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>   				return -1;
>>   		}
>>   	}
>> -
>> +scan:
>>   	for_each_cpu_wrap(cpu, cpus, target + 1) {
>>   		if (has_idle_core) {
>>   			i = select_idle_core(p, cpu, cpus, &idle_cpu);
>>