linux-kernel - Re: [PATCH] sched/fair: Skip newidle_balance() when an idle CPU is woken up to process an IPI

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Date: Mon, 22 Jan 2024 13:59:14 -0800
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: K Prateek Nayak <kprateek.nayak@....com>, linux-kernel@...r.kernel.org
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, bristot@...hat.com,
 vschneid@...hat.com,  gautham.shenoy@....com
Subject: Re: [PATCH] sched/fair: Skip newidle_balance() when an idle CPU is
 woken up to process an IPI

On Fri, 2024-01-19 at 14:15 +0530, K Prateek Nayak wrote:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b803030c3a03..1fedc7e29c98 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8499,6 +8499,16 @@ done: __maybe_unused;
>  	if (!rf)
>  		return NULL;
>  
> +	/*
> +	 * An idle CPU in TIF_POLLING mode might end up here after processing
> +	 * an IPI when the sender sets the TIF_NEED_RESCHED bit and avoids
> +	 * sending an actual IPI. In such cases, where an idle CPU was woken
> +	 * up only to process an interrupt, without necessarily queuing a task
> +	 * on it, skip newidle_balance() to facilitate faster idle re-entry.
> +	 */
> +	if (prev == rq->idle)
> +		return NULL;
> +

Should we check the call function queue directly to detect that there is
an IPI waiting to be processed? something like

	if (!llist_empty(&per_cpu(call_single_queue, rq->cpu)))
		return NULL;

Could there be cases where we want to do idle balance in this code path?
Say a cpu is idle and a scheduling tick came in, we may try
to look for something to run on the idle cpu.  Seems like after
your change above, that would be skipped. 

Tim


>  	new_tasks = newidle_balance(rq, rf);
>  
>  	/*