linux-kernel - Re: [PATCH 2/2] sched/deadline: Don't return invalid cpu in cpudl_maximum

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Date:   Mon, 12 Jun 2017 10:43:08 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Juri Lelli <juri.lelli@....com>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        rostedt@...dmis.org, kernel-team@....com
Subject: Re: [PATCH 2/2] sched/deadline: Don't return invalid cpu in
 cpudl_maximum_cpu()

On Fri, Jun 09, 2017 at 12:42:06PM +0100, Juri Lelli wrote:
> > 
> > This would also work and avoid unnecessary warning. I missed the check
> > to avoid it. https://lkml.org/lkml/2017/3/23/175 was an original patch
> > doing it.
> > 
> > By the way, frankly speaking, I don't like accessing the cpudl instant
> > several times without protection. I rather prefer the following..
> > 
> > But whatever. I like both.
> > 
> > Thnaks,
> > Byungchul
> > 
> > ----->8-----
> > diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> > index 9b314a9..1d369cf 100644
> > --- a/kernel/sched/cpudeadline.c
> > +++ b/kernel/sched/cpudeadline.c
> > @@ -137,11 +137,17 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
> >  	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed)) {
> >  		best_cpu = cpumask_any(later_mask);
> >  		goto out;
> > -	} else if (cpumask_test_cpu(cpudl_maximum_cpu(cp), &p->cpus_allowed) &&
> > -			dl_time_before(dl_se->deadline, cpudl_maximum_dl(cp))) {
> > -		best_cpu = cpudl_maximum_cpu(cp);
> > -		if (later_mask)
> > -			cpumask_set_cpu(best_cpu, later_mask);
> > +	} else {
> > +		int max_cpu = cpudl_maximum_cpu(cp);
> > +		u64 max_dl = cpudl_maximum_dl(cp);
> > +
> > +		if (max_cpu != -1 &&
> > +		    cpumask_test_cpu(max_cpu, &p->cpus_allowed) &&
> > +		    dl_time_before(dl_se->deadline, max_dl)) {
> 
> Don't we access cp 3 times both ways?

I wonder if I miss something.. As you said, in most cases it will, but
I am not sure if we can guarentee that, regardless of arches or
compile optimization level. Sorry if I am wrong.