linux-kernel - Re: [PATCH] sched/fair: Simplify some logic in update_sd_pick

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Date: Fri, 02 Feb 2024 18:40:21 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: David Vernet <void@...ifault.com>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, peterz@...radead.org,
 juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, bristot@...hat.com, kernel-team@...a.com
Subject: Re: [PATCH] sched/fair: Simplify some logic in
 update_sd_pick_busiest()

On 02/02/24 11:07, David Vernet wrote:
> On Fri, Feb 02, 2024 at 06:01:22PM +0100, Valentin Schneider wrote:
>> On 02/02/24 01:02, David Vernet wrote:
>> > When comparing the current struct sched_group with the yet-busiest
>> > domain in update_sd_pick_busiest(), if the two groups have the same
>> > group type, we're currently doing a bit of unnecessary work for any
>> > group >= group_misfit_task. We're comparing the two groups, and then
>> > returning only if false (the group in question is not the busiest).
>> > Othewise, we break, do an extra unnecessary conditional check that's
>> > vacuously false for any group type > group_fully_busy, and then always
>> > return true.
>> >
>> > Let's just return directly in the switch statement instead. This doesn't
>> > change the size of vmlinux with llvm 17 (not surprising given that all
>> > of this is inlined in load_balance()), but it does shrink load_balance()
>> > by 88 bytes on x86. Given that it also improves readability, this seems
>> > worth doing.
>> >
>> > As a bonus, remove an unnecessary goto in update_sd_lb_stats().
>> >
>>
>> Given that's a different scope than what the rest of the patch touches, I'd
>> rather see this as a separate patch.
>>
>> Other than that:
>> Reviewed-by: Valentin Schneider <vschneid@...hat.com>
>
> Thanks, would you like me to send a follow-on series split into two with
> your tag on both? Or were you just letting me know for next time?
>

Well, I'm not picking up any patches, just reviewing them :) So yes I'd say
re-send with the split and feel free to apply the tag on both.

> We could also update this check to only do a strict greater than to
> avoid unnecessary writes, but I figured it was preferable to have no
> logical changes for this iteration:
>
> return sgs->group_misfit_task_load >= busiest->group_misfit_task_load;

That's a good point, I don't think there was a specific reason for going
with a lower-than rather than a lower-or-equal back then:
  cad68e552e77 ("sched/fair: Consider misfit tasks when load-balancing")