linux-kernel - Re: [RESEND, v3, 2/2] sched/deadline: Initialize cp->elements[].cpu to an invalid value

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Date:   Thu, 31 May 2018 23:18:41 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     byungchul park <byungchul.park@....com>
Cc:     peterz@...radead.org, mingo@...nel.org, rostedt@...dmis.org,
        tglx@...utronix.de, raistlin@...ux.it,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        bristot@...hat.com, kernel-team@....com
Subject: Re: [RESEND, v3, 2/2] sched/deadline: Initialize cp->elements[].cpu
 to an invalid value

On Thu, May 31, 2018 at 11:12:55PM -0700, Joel Fernandes wrote:
> On Mon, Jan 08, 2018 at 03:14:41PM +0900, byungchul park wrote:
> > Currently, migrating tasks to cpu0 unconditionally happens when the
> > heap is empty, since cp->elements[].cpu was initialized to 0(=cpu0).
> > We have to distinguish between the empty case and cpu0 to avoid the
> > unnecessary migrations. Therefore, it has to return an invalid value
> > e.i. -1 in that case.
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > Acked-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > Acked-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> > ---
> >  kernel/sched/cpudeadline.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> > index 9f02035..bcf903f 100644
> > --- a/kernel/sched/cpudeadline.c
> > +++ b/kernel/sched/cpudeadline.c
> > @@ -138,6 +138,12 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
> >  		int best_cpu = cpudl_maximum_cpu(cp);
> >  		WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
> >  
> > +		/*
> > +		 * The heap tree is empty for now, just return.
> > +		 */
> > +		if (best_cpu == -1)
> > +			return 0;
> > +
> >  		if (cpumask_test_cpu(best_cpu, &p->cpus_allowed) &&
> >  		    dl_time_before(dl_se->deadline, cpudl_maximum_dl(cp))) {
> >  			if (later_mask)
> > @@ -265,8 +271,10 @@ int cpudl_init(struct cpudl *cp)
> >  		return -ENOMEM;
> >  	}
> >  
> > -	for_each_possible_cpu(i)
> > +	for_each_possible_cpu(i) {
> > +		cp->elements[i].cpu = -1;
> >  		cp->elements[i].idx = IDX_INVALID;
> 
> Shouldn't you also set cp->elements[cpu].cpu to -1 in cpudl_clear (when you
> set cp->elements[cpu].cpu to IDX_INVALID there)?

I messed up my words, I meant : "when setting cp->elements[cpu].idx to
IDX_INVALID there". Which means I need to call it a day :-)

- Joel