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Message-ID: <b4f5ac150685456cf45a342e3bb1f28cdd557a53.camel@linux.intel.com>
Subject: Re: [PATCH] sched/fair: Skip newidle_balance() when an idle CPU is
 woken up to process an IPI
From: Tim Chen <tim.c.chen@linux.intel.com>
To: K Prateek Nayak <kprateek.nayak@amd.com>, linux-kernel@vger.kernel.org
Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, 
 vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, 
 bsegall@google.com, mgorman@suse.de, bristot@redhat.com,
 vschneid@redhat.com,  gautham.shenoy@amd.com
Date: Mon, 22 Jan 2024 13:59:14 -0800
In-Reply-To: <20240119084548.2788-1-kprateek.nayak@amd.com>
References: <20240119084548.2788-1-kprateek.nayak@amd.com>
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On Fri, 2024-01-19 at 14:15 +0530, K Prateek Nayak wrote:
>=20
> 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;
> =20
> +	/*
> +	 * 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 =3D=3D 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.=20

Tim


>  	new_tasks =3D newidle_balance(rq, rf);
> =20
>  	/*