linux-kernel - Re: [PATCH v5 1/2] sched/deadline: optimize sched_dl_global

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Date:   Thu, 8 Oct 2020 21:58:07 +0800
From:   Peng Liu <iwtbavbm@...il.com>
To:     Peter Zijlstra <peterz@...radead.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, valentin.schneider@....com, raistlin@...ux.it,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/2] sched/deadline: optimize
 sched_dl_global_validate()

On Wed, Oct 07, 2020 at 06:55:33PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 11:12:29PM +0800, Peng Liu wrote:
> > +/* Used for dl_bw check and update. */
> > +static u32 dl_generation;
> 
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 28709f6b0975..53477e8b26b0 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -798,6 +798,13 @@ struct root_domain {
> >  	 */
> >  	cpumask_var_t		dlo_mask;
> >  	atomic_t		dlo_count;
> > +
> > +	/*
> > +	 * Indicate whether a root_domain's dl_bw has been checked or
> > +	 * updated. It's monotonously increasing, then wrap around.
> > +	 */
> > +	u32 visit_gen;
> > +
> >  	struct dl_bw		dl_bw;
> >  	struct cpudl		cpudl;
> >  
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index dd7770226086..90f3e5558fa2 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -516,6 +516,7 @@ static int init_rootdomain(struct root_domain *rd)
> >  	init_irq_work(&rd->rto_push_work, rto_push_irq_work_func);
> >  #endif
> >  
> > +	rd->visit_gen = 0;
> >  	init_dl_bw(&rd->dl_bw);
> >  	if (cpudl_init(&rd->cpudl) != 0)
> >  		goto free_rto_mask;
> 
> I'm fairly sure I made the generation a u64, the above is susceptible to
> a false positive due to wrap-around.
> 
> Increase the generation to -1, create a new root domain, then the next
> generation is 0 and we'll skip the new domain, even though it should be
> updated.

Ah... at first, I also thought that u32 is "big enough" given that
no one would frequently change the settings, 'wrap-around' shouldn't
be a concern.

So...OK, I will revert it back to u64. What a big circle! :)

Thanks for your time!