Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

From: "Li, Aubrey" <aubrey.li@linux.intel.com>
To: Vincent Guittot <vincent.guittot@linaro.org>,
	Mel Gorman <mgorman@techsingularity.net>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Barry Song <song.bao.hua@hisilicon.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Ziljstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Linux-ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched
Date: Fri, 4 Dec 2020 21:47:28 +0800	[thread overview]
Message-ID: <c384f484-d74b-a114-2a34-5428b58ca5d2@linux.intel.com> (raw)
In-Reply-To: <3d8a6d19-afac-dc93-127d-da6505402cdf@linux.intel.com>

On 2020/12/4 21:40, Li, Aubrey wrote:
> On 2020/12/4 21:17, Vincent Guittot wrote:
>> On Fri, 4 Dec 2020 at 14:13, Vincent Guittot <vincent.guittot@linaro.org> wrote:
>>>
>>> On Fri, 4 Dec 2020 at 12:30, Mel Gorman <mgorman@techsingularity.net> wrote:
>>>>
>>>> On Fri, Dec 04, 2020 at 11:56:36AM +0100, Vincent Guittot wrote:
>>>>>> The intent was that the sibling might still be an idle candidate. In
>>>>>> the current draft of the series, I do not even clear this so that the
>>>>>> SMT sibling is considered as an idle candidate. The reasoning is that if
>>>>>> there are no idle cores then an SMT sibling of the target is as good an
>>>>>> idle CPU to select as any.
>>>>>
>>>>> Isn't the purpose of select_idle_smt ?
>>>>>
>>>>
>>>> Only in part.
>>>>
>>>>> select_idle_core() looks for an idle core and opportunistically saves
>>>>> an idle CPU candidate to skip select_idle_cpu. In this case this is
>>>>> useless loops for select_idle_core() because we are sure that the core
>>>>> is not idle
>>>>>
>>>>
>>>> If select_idle_core() finds an idle candidate other than the sibling,
>>>> it'll use it if there is no idle core -- it picks a busy sibling based
>>>> on a linear walk of the cpumask. Similarly, select_idle_cpu() is not
>>>
>>> My point is that it's a waste of time to loop the sibling cpus of
>>> target in select_idle_core because it will not help to find an idle
>>> core. The sibling  cpus will then be check either by select_idle_cpu
>>> of select_idle_smt
>>
>> also, while looping the cpumask, the sibling cpus of not idle cpu are
>> removed and will not be check
>>
> 
> IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)?
> If the target's sibling is removed from select_idle_mask from select_idle_core(),
> select_idle_cpu() will lose the chance to pick it up?

aha, no, select_idle_mask will be re-assigned in select_idle_cpu() by:

	cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);

So, yes, I guess we can remove the cpu_smt_mask(target) from select_idle_core() safely.

> 
> Thanks,
> -Aubrey
> 