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Subject: Re: [PATCH] sched/fair: Make sched-idle cpu selection consistent
 throughout
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org
References: <5eba2fb4af9ebc7396101bb9bd6c8aa9c8af0710.1571899508.git.viresh.kumar@linaro.org>
 <7d3a1549-a99c-ae42-6074-8ed2ecd7074f@linux.ibm.com>
 <20191025081108.6gaprbwm5fvokun6@vireshk-i7>
From:   Parth Shah <parth@linux.ibm.com>
Date:   Fri, 25 Oct 2019 17:30:34 +0530
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On 10/25/19 1:41 PM, Viresh Kumar wrote:
> On 25-10-19, 12:13, Parth Shah wrote:
>> Hi Viresh,
>>
>> On 10/24/19 12:15 PM, Viresh Kumar wrote:
>>> There are instances where we keep searching for an idle CPU despite
>>> having a sched-idle cpu already (in find_idlest_group_cpu(),
>>> select_idle_smt() and select_idle_cpu() and then there are places where
>>> we don't necessarily do that and return a sched-idle cpu as soon as we
>>> find one (in select_idle_sibling()). This looks a bit inconsistent and
>>> it may be worth having the same policy everywhere.
>>>
>>> On the other hand, choosing a sched-idle cpu over a idle one shall be
>>> beneficial from performance point of view as well, as we don't need to
>>> get the cpu online from a deep idle state which is quite a time
>>> consuming process and delays the scheduling of the newly wakeup task.
>>>
>>> This patch tries to simplify code around sched-idle cpu selection and
>>> make it consistent throughout.
>>>
>>> FWIW, tests were done with the help of rt-app (8 SCHED_OTHER and 5
>>> SCHED_IDLE tasks, not bound to any cpu) on ARM platform (octa-core), and
>>> no significant difference in scheduling latency of SCHED_OTHER tasks was
>>> found.
>>>
>>> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
>>> ---
>>
>> [...]
>>
>>> @@ -5755,13 +5749,11 @@ static int select_idle_smt(struct task_struct *p, int target)
>>>  	for_each_cpu(cpu, cpu_smt_mask(target)) {
>>>  		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>>>  			continue;
>>> -		if (available_idle_cpu(cpu))
>>> +		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
>>>  			return cpu;
>>
>> I guess this is a correct approach, but just wondering what if we still
>> keep searching for a sched_idle CPU even though we have found an
>> available_idle CPU?
> 
> I do believe selecting a sched-idle CPU should almost always be better
> (performance wise), unless we have a strong argument against it. And
> anyway, the load balancer will get triggered at a later point of time
> and will pull away these newly wakeup tasks to idle CPUs. The
> advantage we get out of it is that the tasks get serviced a bit
> earlier when they first get queued.
> 
> It is really up to the maintainers to see what kind of policy do we
> want to adapt here and not a choice I can make :)
> 

yeah, I agree. I will favor selecting sched-idle first for smaller domains
like SMT but would leave on experts.
BTW, if sched-idle is given priority then maybe...
> @@ -5818,13 +5810,11 @@ static int select_idle_cpu(struct task_struct *p,
> struct sched_domain *sd, int t
>
>  	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
>  		if (!--nr)
> -			return si_cpu;
> +			return -1;
>  		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>  			continue;
> -		if (available_idle_cpu(cpu))
> +		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
>  			break;
...here too can be optimized I guess.


Thanks,
Parth