linux-kernel - Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup

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Date:   Fri, 17 Sep 2021 11:44:00 +0800
From:   Barry Song <21cnbao@...il.com>
To:     aubrey.li@...ux.intel.com
Cc:     linux-kernel@...r.kernel.org, mgorman@...hsingularity.net,
        mingo@...nel.org, peterz@...radead.org, song.bao.hua@...ilicon.com,
        valentin.schneider@....com, vincent.guittot@...aro.org,
        yangyicong@...wei.com
Subject: Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup

[Aubrey wrote]
> The idle core information is already in idle cpu mask, do we have a quick
> and cheap way to extract out?

seems hard.

since we need an intermediate cpumask memory to save the result of cpumask
bitmap operations, the only way i can get the idle core and make sure "cpus"
isn't broken is:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 361927803e31..abd844bcfb86 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6187,8 +6187,14 @@ void __update_idle_core(struct rq *rq)
  */
 static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
 {
-       bool idle = true;
-       int cpu;
+       bool idle = true, possible_idle;
+       int cpu, wb, wa;
+
+       wb = cpumask_weight(cpus);
+       cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
+       wa = cpumask_weight(cpus);
+
+       possible_idle = (wa - wb == cpumask_weight(cpu_smt_mask(core)));
 
        if (!static_branch_likely(&sched_smt_present))
                return __select_idle_cpu(core, p);
@@ -6212,7 +6218,6 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
        if (idle)
                return core;
 
-       cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
        return -1;
 }

but the questions are:
1. is it really cheap?
2. how to use possible_idle? right now, select_idle_core() is also selecting idle cpu. we shouldn't simply return when possible_idle is false.

Thanks
Barry