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

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Message-ID: <jhj4ktjpw2z.mognet@arm.com>
Date:   Thu, 16 Apr 2020 11:24:20 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v3 9/9] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan


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?

>> +               break;
>> +       case WF_FORK:
>> +               sd_flag = SD_BALANCE_FORK;
>> +               sd = rcu_dereference(per_cpu(sd_balance_fork, cpu));
>> +               break;
>> +       default:
>> +               sd_flag = SD_BALANCE_EXEC;
>> +               sd = rcu_dereference(per_cpu(sd_balance_exec, cpu));
>> +       }
>>
>>         /*
>>          * If !want_affine, we just look for the highest domain where