linux-kernel - Re: [PATCH 1/9] sched: Simplify get_nohz_timer

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Message-ID: <20230809194822.GV212435@hirez.programming.kicks-ass.net>
Date:   Wed, 9 Aug 2023 21:48:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joel@...lfernandes.org>
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, vschneid@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/9] sched: Simplify get_nohz_timer_target()

On Sun, Aug 06, 2023 at 05:39:24PM -0400, Joel Fernandes wrote:
> On Sun, Aug 6, 2023 at 9:52 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Use guards to reduce gotos and simplify control flow.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/sched/core.c |   15 ++++++---------
> >  1 file changed, 6 insertions(+), 9 deletions(-)
> >
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1097,25 +1097,22 @@ int get_nohz_timer_target(void)
> >
> >         hk_mask = housekeeping_cpumask(HK_TYPE_TIMER);
> >
> > -       rcu_read_lock();
> > +       guard(rcu)();
> > +
> >         for_each_domain(cpu, sd) {
> >                 for_each_cpu_and(i, sched_domain_span(sd), hk_mask) {
> >                         if (cpu == i)
> >                                 continue;
> >
> > -                       if (!idle_cpu(i)) {
> > -                               cpu = i;
> > -                               goto unlock;
> > -                       }
> > +                       if (!idle_cpu(i))
> > +                               return i;
> >                 }
> >         }
> >
> >         if (default_cpu == -1)
> >                 default_cpu = housekeeping_any_cpu(HK_TYPE_TIMER);
> > -       cpu = default_cpu;
> > -unlock:
> > -       rcu_read_unlock();
> > -       return cpu;
> > +
> > +       return default_cpu;
> >  }
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> I haven't looked into the actual implementation of the guard stuff,
> but rcu_read_lock_guarded() is less of an eyesore to me than
> guard(rcu)(); TBH.

I readily admit it isn't the prettiest construct, my brain is warped by
many years of C++ and I can read it as: guard<rcu>(), but I'm not sure
that's actually better :-)

The advantage of all this is that you also get:

	scoped_guard (rcu) {
	}

for 'free'.