linux-kernel - Re: [PATCH 02/10] sched: Factor out code to should_we

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Date:	Fri, 23 Aug 2013 04:37:35 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Lei Wen <leiwen@...vell.com>, Rik van Riel <riel@...riel.com>,
	Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH 02/10] sched: Factor out code to should_we_balance()

On Thu, Aug 22, 2013 at 3:42 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Aug 22, 2013 at 02:58:27AM -0700, Paul Turner wrote:
>> On Mon, Aug 19, 2013 at 9:01 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
>> > +               if (local_group)
>> >                         load = target_load(i, load_idx);
>>
>> Keep the braces here:
>>
>>   if (local_group) {
>>     load = target_load(i, load_idx);
>>   } else {
>>    ...
>
> Right you are, I misplaced that hunk in the next patch.
>
>> > -               } else {
>> > +               else {
>> >                         load = source_load(i, load_idx);
>> >                         if (load > max_cpu_load)
>> >                                 max_cpu_load = load;
>
>
>> > @@ -5123,12 +5120,11 @@ static int load_balance(int this_cpu, st
>> >
>> >         schedstat_inc(sd, lb_count[idle]);
>> >
>> > -redo:
>> > -       group = find_busiest_group(&env, balance);
>> > -
>> > -       if (*balance == 0)
>> > +       if (!(*should_balance = should_we_balance(&env)))
>> >                 goto out_balanced;
>>
>> Given we always initialize *should_balance where we care, it might be
>> more readable to write this as:
>
> Ah, but we don't actually, idle_balance() doesn't initialize
> should_balance -- easy enough to fix though.

I should have been more explicit here.

idle_balance() doesn't initialize it, but it completely ignores the
result; it's just passing something in case a write occurs.
(Arguably it should be int unused = 1 instead of int balance = 1)

>
>> if (!should_we_balance(&env)) {
>>   *continue_balancing = 0;
>>   goto out_balanced;
>> }
>>
>> [ With a rename to can_balance ]
>
> You confused me, your code implied
> %s/should_balance/continue_balancing/g but now you suggest
> %%s/should_balance/can_balance/g ?
>
> I'm fine with either.

My bad, I started typing this comment, then went and looked at other
parts of the patch and came back to it :)

I think continue_balancing is more clear.

>
>> >
>> > +redo:
>>
>> One behavioral change worth noting here is that in the redo case if a
>> CPU has become idle we'll continue trying to load-balance in the
>> !new-idle case.
>>
>> This could be unpleasant in the case where a package has a pinned busy
>> core allowing this and a newly idle cpu to start dueling for load.
>>
>> While more deterministically bad in this case now, it could racily do
>> this before anyway so perhaps not worth worrying about immediately.
>
> Ah, because the old code would effectively redo the check and find the
> idle cpu and thereby our cpu would no longer be the balance_cpu.
>
> Indeed. And I don't think this was an intentional change. I'll go put
> the redo back before should_we_balance().
>

I was trying to get through the rest of these today but I'm out of
time.  I should be able to finish reviewing the other patches in the
set tomorrow.
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