linux-kernel - Re: [PATCH 5/9] sched: Simplify ttwu()

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Date:   Wed, 9 Aug 2023 21:26:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] sched: Simplify ttwu()

On Wed, Aug 09, 2023 at 04:21:36PM +0100, Valentin Schneider wrote:
> On 01/08/23 22:41, Peter Zijlstra wrote:
> > Use guards to reduce gotos and simplify control flow.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/sched/core.c |  221 +++++++++++++++++++++++++---------------------------
> >  1 file changed, 109 insertions(+), 112 deletions(-)
> >
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3706,14 +3706,14 @@ ttwu_stat(struct task_struct *p, int cpu
> >               struct sched_domain *sd;
> >
> >               __schedstat_inc(p->stats.nr_wakeups_remote);
> > -		rcu_read_lock();
> > +
> > +		guard(rcu)();
> 
> This isn't strictly equivalent, right? AFAICT that pushes the
> rcu_read_unlock() further down than it currently is - not a big deal, but
> indentation aside scoped_guard() would preserve that.

The full hunk:

| @@ -3706,14 +3706,14 @@ ttwu_stat(struct task_struct *p, int cpu
|  		struct sched_domain *sd;
|  
|  		__schedstat_inc(p->stats.nr_wakeups_remote);
| -		rcu_read_lock();
| +
| +		guard(rcu)();
|  		for_each_domain(rq->cpu, sd) {
|  			if (cpumask_test_cpu(cpu, sched_domain_span(sd))) {
|  				__schedstat_inc(sd->ttwu_wake_remote);
|  				break;
|  			}
|  		}
| -		rcu_read_unlock();
|  	}

And you'll see the guard goes out of scope here ^

Which is the exact place rcu_read_unlock() was at, no?

|  
|  	if (wake_flags & WF_MIGRATED)