linux-kernel - Re: [PATCH v6] sched_ext: idle: Refresh idle masks during idle-to-idle transitions

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Message-ID: <Z4F3Xcgl9CUhHVyn@slm.duckdns.org>
Date: Fri, 10 Jan 2025 09:39:09 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6] sched_ext: idle: Refresh idle masks during
 idle-to-idle transitions

Hello, Andrea.

On Fri, Jan 10, 2025 at 09:46:25AM +0100, Andrea Righi wrote:
...
> +	if (do_notify) {
> +		if (SCX_HAS_OP(update_idle) && !scx_rq_bypassing(rq))
> +			SCX_CALL_OP(SCX_KF_REST, update_idle, cpu_of(rq), idle);
> +	} else {
> +		bool is_prev_idle;
> +
> +		/* Refresh idle masks during idle-to-idle transitions */

Can you add a bit more explanation on what case this path is handling here
or in the function comment? The function comment explains what it's about
but doesn't quite explain the exact sequence which isn't very intuitive.

> +		rcu_read_lock();
> +		is_prev_idle = is_idle_task(rcu_dereference(rq->curr));
> +		rcu_read_unlock();
> +
> +		if (!is_prev_idle)
>  			return;

This function is always called under the rq lock, right? We can assert that
and skip the rcu dancing.

>  	}
>  
> +	if (!static_branch_likely(&scx_builtin_idle_enabled))
> +		return;

Would structure like the following be better? It makes clear that the last
condition checks are for the builtin idle path.

        if (SCX_HAS_OP(update_idle) && do_notify && !scx_rq_bypassing(rq))
                // call ops.update_idle().

        if (!scx_builtin_idle_enabled || (!do_notify && !is_idle_task(rq->curr)))
                return;

Thanks.

-- 
tejun