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From:   Byungchul Park <max.byungchul.park@gmail.com>
Date:   Sun, 3 Jun 2018 14:20:32 +0900
Message-ID: <CANrsvRPaE1oDdD5JyxsCo7z-W=yBycOdDMf-+Tam3rMKB+MHmA@mail.gmail.com>
Subject: Re: [RESEND, v3, 2/2] sched/deadline: Initialize cp->elements[].cpu
 to an invalid value
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Byungchul Park <byungchul.park@lge.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, rostedt@goodmis.org,
        Thomas Gleixner <tglx@linutronix.de>, raistlin@linux.it,
        linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@gmail.com>,
        bristot@redhat.com, kernel-team@lge.com, kernel-team@android.com
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On Sat, Jun 2, 2018 at 12:52 AM, Joel Fernandes <joel@joelfernandes.org> wrote:
> On Fri, Jun 01, 2018 at 04:10:56PM +0900, Byungchul Park wrote:
>> > 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@lge.com>
>> > > > Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
>> > > > Acked-by: Daniel Bristot de Oliveira <bristot@redhat.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 :-)
>>
>> Ah.. I agree with you. It might be a problem when removing the last
>> element.. Then I think the following change should also be applied
>> additionally. Right?
>
> Yes.
>
>> ---
>> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
>> index 8d9562d..44d4c88 100644
>> --- a/kernel/sched/cpudeadline.c
>> +++ b/kernel/sched/cpudeadline.c
>> @@ -172,12 +172,14 @@ void cpudl_clear(struct cpudl *cp, int cpu)
>>         } else {
>>                 new_cpu = cp->elements[cp->size - 1].cpu;
>>                 cp->elements[old_idx].dl = cp->elements[cp->size - 1].dl;
>> -               cp->elements[old_idx].cpu = new_cpu;
>> +               cp->elements[old_idx].cpu = (new_cpu == cpu) ? -1 : new_cpu;
>>                 cp->size--;
>> -               cp->elements[new_cpu].idx = old_idx;
>>                 cp->elements[cpu].idx = IDX_INVALID;
>> -               cpudl_heapify(cp, old_idx);
>>
>> +               if (new_cpu != cpu) {
>> +                       cp->elements[new_cpu].idx = old_idx;
>> +                       cpudl_heapify(cp, old_idx);
>> +               }
>>                 cpumask_set_cpu(cpu, cp->free_cpus);
>
> This looks a bit confusing. How about the following? (untested)

Hello,

Whatever. Your code also looks good to me.

I just wanna follow the maintainers' decision. ;)

> thanks,
>
>  - Joel
>
> ---8<-----------------------
>
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index 50316455ea66..741a97e58c05 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -129,6 +129,10 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>         } else {
>                 int best_cpu = cpudl_maximum(cp);
>
> +               /* The max-heap is empty, just return. */
> +               if (best_cpu == -1)
> +                       return 0;
> +
>                 WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
>
>                 if (cpumask_test_cpu(best_cpu, &p->cpus_allowed) &&
> @@ -167,6 +171,12 @@ void cpudl_clear(struct cpudl *cp, int cpu)
>                  * This could happen if a rq_offline_dl is
>                  * called for a CPU without -dl tasks running.
>                  */
> +       } else if (cp->size == 1) {
> +               /* Only one element in max-heap, clear it */
> +               cp->elements[0].cpu = -1;
> +               cp->elements[cpu].idx = IDX_INVALID;
> +               cp->size--;
> +               cpumask_set_cpu(cpu, cp->free_cpus);
>         } else {
>                 new_cpu = cp->elements[cp->size - 1].cpu;
>                 cp->elements[old_idx].dl = cp->elements[cp->size - 1].dl;
> @@ -262,6 +272,9 @@ int cpudl_init(struct cpudl *cp)
>         for_each_possible_cpu(i)
>                 cp->elements[i].idx = IDX_INVALID;
>
> +       /* Mark heap as initially empty */
> +       cp->elements[0].cpu = -1;
> +
>         return 0;
>  }
>


-- 
Thanks,
Byungchul