linux-kernel - Re: [PATCH v3 9/9] sched/topology: Define and use shortcut pointers for wakeup sd

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Date:   Thu, 16 Apr 2020 15:04:22 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 9/9] sched/topology: Define and use shortcut pointers
 for wakeup sd_flag scan

On 16.04.20 12:24, Valentin Schneider wrote:
> 
> On 16/04/20 08:46, Vincent Guittot wrote:
>>> @@ -6657,7 +6646,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>>>
>>>         rcu_read_lock();
>>>
>>> -       sd = highest_flag_domain(cpu, sd_flag);
>>> +       switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
>>> +       case WF_TTWU:
>>> +               sd_flag = SD_BALANCE_WAKE;
>>> +               sd = rcu_dereference(per_cpu(sd_balance_wake, cpu));
>>
>> It's worth having a direct pointer for the fast path which we always
>> try to keep short but the other paths are already slow and will not
>> get any benefit of this per cpu pointer.
>> We should keep the loop for the slow paths
>>
> 
> Which fast/slow paths are you referring to here? want_affine vs
> !want_affine? If so, do you then mean that we should do the switch case
> only when !want_affine, and otherwise look for the domain via the
> for_each_domain() loop?

Coming back to the v2 discussion on this patch

https://lore.kernel.org/r/20200311181601.18314-10-valentin.schneider@arm.com

SD_BALANCE_WAKE is not used in mainline anymore, so wakeups are always
fast today.

I.e. you wouldn't need a per_cpu(sd_balance_wake, cpu) since it's always
NULL.

I.e. want_affine logic and the 'for_each_domain(cpu, tmp)' isn't needed
anymore.

This will dramatically simplify the code in select_task_rq_fair().

But I guess Vincent wants to keep the functionality so we're able to
enable SD_BALANCE_WAKE on certain sd's?