linux-kernel - Re: [PATCH v3 3/6] sched: Change wait_task_inactive()s match

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Date:   Wed, 7 Sep 2022 09:23:44 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, oleg@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        ebiederm@...ssion.com, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 3/6] sched: Change wait_task_inactive()s match_state


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Sun, Sep 04, 2022 at 12:44:36PM +0200, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > Make wait_task_inactive()'s @match_state work like ttwu()'s @state.
> > > 
> > > That is, instead of an equal comparison, use it as a mask. This allows
> > > matching multiple block conditions.
> > > 
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > >  kernel/sched/core.c |    4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -3295,7 +3295,7 @@ unsigned long wait_task_inactive(struct
> > >  		 * is actually now running somewhere else!
> > >  		 */
> > >  		while (task_running(rq, p)) {
> > > -			if (match_state && unlikely(READ_ONCE(p->__state) != match_state))
> > > +			if (match_state && !(READ_ONCE(p->__state) & match_state))
> > >  				return 0;
> > 
> > We lose the unlikely annotation there - but I guess it probably never 
> > really mattered anyway?
> 
> So any wait_task_inactive() caller does want that case to be true, but
> the whole match_state precondition mostly wrecks things anyway. If
> anything it should've been:
> 
> 		if (likely(match_state && !(READ_ONCE(p->__state) & match_state)))
> 			return 0;
> 
> but I can't find it in me to care too much here.

Yeah, I agree that this is probably the most likely branch - and default 
compiler code generation behavior should be pretty close to that to begin 
with.

Ie. ack on dropping the unlikely() annotation. :-)

Might make sense to add a sentence to the changelog though, in case anyone 
(like me) is wondering about whether the dropped annotation was intended.

Thanks,

	Ingo