linux-kernel - Re: [PATCH 0/5] sched: Lazy preemption muck

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Message-ID: <20241009080202.GJ17263@noisy.programming.kicks-ass.net>
Date: Wed, 9 Oct 2024 10:02:02 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, tglx@...utronix.de,
	mingo@...nel.org, linux-kernel@...r.kernel.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, efault@....de
Subject: Re: [PATCH 0/5] sched: Lazy preemption muck

On Wed, Oct 09, 2024 at 08:20:19AM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-10-08 21:40:05 [-0700], Ankur Arora wrote:
> > > While comparing this vs what I have:
> > > - need_resched()
> > >   It checked both (tif_need_resched_lazy() || tif_need_resched()) while
> > >   now it only looks at tif_need_resched().
> > >   Also ensured that raw_irqentry_exit_cond_resched() does not trigger on
> > >   lazy.
> > >   I guess you can argue both ways what makes sense, just noting…
> > 
> > I think we want need_resched() to be only tif_need_resched(). That way
> > preemption in lazy mode *only* happens at the user mode boundary.
> 
> There are places such as __clear_extent_bit() or select_collect() where
> need_resched() is checked and if 0 they loop again. For these kind of
> users it would probably make sense to allow them to preempt themself.
> We could also add a new function which checks both and audit all users
> and check what would make sense base on $criteria.

Do we really need this -- wasn't the idea to have thing 'delay' until
the actual NEED_RESCHED bit gets set?