linux-kernel - Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle

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Date:   Sat, 10 Mar 2018 12:41:47 -0800
From:   Rohit Jain <rohit.k.jain@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        joelaf@...gle.com, dietmar.eggemann@....com,
        vincent.guittot@...aro.org, morten.rasmussen@....com,
        eas-dev@...ts.linaro.org
Subject: Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in
 select_idle_sibling

Hi Peter,

On 02/09/2018 04:53 AM, Peter Zijlstra wrote:

<snip>

>>   	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>>   	if (!this_sd)
>> @@ -6173,8 +6183,15 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>>   			return -1;
>>   		if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
>>   			continue;
>> +		if (idle_cpu(cpu)) {
>> +			if (full_capacity(cpu)) {
>> +				best_cpu = cpu;
>> +				break;
>> +			} else if (capacity_of(cpu) > best_cap) {
>> +				best_cap = capacity_of(cpu);
>> +				best_cpu = cpu;
>> +			}
>> +		}
> No need for the else. And you'll note you're once again inconsistent
> with your previous self.
>
> But here I worry about big.little a wee bit. I think we're allowed big
> and little cores on the same L3 these days, and you can't directly
> compare capacity between them.
>
>
<snip>

After pulling to the latest code I see that the changes by Mel Gorman
(commit 32e839dda3ba576943365f0f5817ce5c843137dc) have created a short
path for returning an idle CPU.

The fact that now there exists a short path, to bypass rest of
select_idle_sibling (SIS) is causing a regression in the
"hackbench + ping" testcase *when* I add capacity awareness in the baseline
code as was discussed here.

In details: baseline today has a short cut in the recent_used_cpu to
bypass SIS. When I add capacity awareness in the SIS code path, causing
that extra search to find a better CPU itself is taking more time than
the benefit it provides.

However, there are certain patches which reduce SIS cost while
maintaining a similar spread for threads on CPUs. When I use those
patches I see that the benefit for adding capacity awareness is
restored. Please suggest how to proceed on this.

Thanks,
Rohit