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

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Message-ID: <20241009100133.2569e2a7@gandalf.local.home>
Date: Wed, 9 Oct 2024 10:01:33 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 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, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, efault@....de
Subject: Re: [PATCH 0/5] sched: Lazy preemption muck

On Wed, 9 Oct 2024 10:02:02 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> 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?

If we think about it as what would happen with the current PREEMPT_NONE,
wouldn't need_resched() return true as soon as NEED_RESCHED is set? That
means, with PREEMPT_AUTO, it should return true if LAZY_NEED_RESCHED is
set, right? That would make it act the same in both cases.

-- Steve