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From:   Barry Song <21cnbao@gmail.com>
To:     aubrey.li@linux.intel.com
Cc:     linux-kernel@vger.kernel.org, mgorman@techsingularity.net,
        mingo@kernel.org, peterz@infradead.org, song.bao.hua@hisilicon.com,
        valentin.schneider@arm.com, vincent.guittot@linaro.org,
        yangyicong@huawei.com
Subject: Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup
Date:   Fri, 17 Sep 2021 12:15:39 +0800
Message-Id: <20210917041539.7862-1-21cnbao@gmail.com>
In-Reply-To: <125eb98a-241b-078f-1844-b0521425ed1e@linux.intel.com>
References: <125eb98a-241b-078f-1844-b0521425ed1e@linux.intel.com>
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> @@ -4965,6 +4965,7 @@ void scheduler_tick(void)
>
>  #ifdef CONFIG_SMP
> 	rq->idle_balance = idle_cpu(cpu);
> +	update_idle_cpumask(cpu, rq->idle_balance);
>  	trigger_load_balance(rq);
>  #endif
> }

might be stupid, a question bothering yicong and me is that why don't we
choose to update_idle_cpumask() while idle task exits and switches to a
normal task?
for example, before tick comes, cpu has exited from idle, but we are only
able to update it in tick. this makes idle_cpus_span inaccurate, thus we
will scan cpu which isn't actually idle in select_idle_sibling.
is it because of the huge update overhead?

Thanks
barry