linux-kernel - Re: [PATCH 2/8] sched/fair: Limit hrtick work

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Message-ID: <20251128082517.GA3245006@noisy.programming.kicks-ass.net>
Date: Fri, 28 Nov 2025 09:25:17 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: tglx@...utronix.de, arnd@...db.de, anna-maria@...utronix.de,
	frederic@...nel.org, luto@...nel.org, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
	oliver.sang@...el.com
Subject: Re: [PATCH 2/8] sched/fair: Limit hrtick work

On Fri, Sep 19, 2025 at 08:29:09PM +0530, K Prateek Nayak wrote:
> Hello Peter,
> 
> On 9/18/2025 1:22 PM, Peter Zijlstra wrote:
> > @@ -13119,6 +13119,12 @@ static void task_tick_fair(struct rq *rq
> >  		entity_tick(cfs_rq, se, queued);
> >  	}
> >  
> > +	if (queued) {
> > +		if (!need_resched())
> > +			hrtick_start_fair(rq, curr);
> 
> Do we need a hrtick_start_fair() here? Queued tick will always do a
> resched_curr_lazy() - if another HRTICK fires before the next tick,
> all it'll do is resched_curr_lazy() again and the next opportunity to
> resched is either exit to userspace or the periodic tick firing and
> promoting that LAZY to a full NEED_RESCHED.

I think I had a version where entity_tick() doesn't force need_resched
on queue, and in that case the timer, which is wallclock, and
update_curr(), which is task_clock, might disagree and we might not have
reached the deadline, and so we need to try again.