linux-kernel - Re: [RFC PATCH v3] sched/fair: select idle cpu from idle cpumask for task wakeup

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Date:   Thu, 12 Nov 2020 20:12:01 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        valentin.schneider@....com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...el.com>,
        Jiang Biao <benbjiang@...il.com>
Subject: Re: [RFC PATCH v3] sched/fair: select idle cpu from idle cpumask for
 task wakeup

On 2020/11/12 18:57, Qais Yousef wrote:
> On 10/21/20 23:03, Aubrey Li wrote:
>> From: Aubrey Li <aubrey.li@...el.com>
>>
>> Added idle cpumask to track idle cpus in sched domain. When a CPU
>> enters idle, its corresponding bit in the idle cpumask will be set,
>> and when the CPU exits idle, its bit will be cleared.
>>
>> When a task wakes up to select an idle cpu, scanning idle cpumask
>> has low cost than scanning all the cpus in last level cache domain,
>> especially when the system is heavily loaded.
>>
>> v2->v3:
>> - change setting idle cpumask to every idle entry, otherwise schbench
>>   has a regression of 99th percentile latency.
>> - change clearing idle cpumask to nohz_balancer_kick(), so updating
>>   idle cpumask is ratelimited in the idle exiting path.
>> - set SCHED_IDLE cpu in idle cpumask to allow it as a wakeup target.
>>
>> v1->v2:
>> - idle cpumask is updated in the nohz routines, by initializing idle
>>   cpumask with sched_domain_span(sd), nohz=off case remains the original
>>   behavior.
> 
> Did you intend to put the patch version history in the commit message?
> 
> I started looking at this last week but got distracted. I see you already got
> enough reviews, so my 2p is that I faced some compilation issues:
> 
> 	aarch64-linux-gnu-ld: kernel/sched/idle.o: in function `set_next_task_idle':
> 	/mnt/data/src/linux/kernel/sched/idle.c:405: undefined reference to `update_idle_cpumask'
> 	aarch64-linux-gnu-ld: kernel/sched/fair.o: in function `nohz_balancer_kick':
> 	/mnt/data/src/linux/kernel/sched/fair.c:10150: undefined reference to `update_idle_cpumask'
> 	aarch64-linux-gnu-ld: /mnt/data/src/linux/kernel/sched/fair.c:10148: undefined reference to `update_idle_cpumask'
> 
> Because of the missing CONFIG_SCHED_SMT in my .config. I think
> update_idle_cpumask() should be defined unconditionally.

Thanks to point this out timely, :), I'll fix it in the next version.

-Aubrey