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From: Barry Song <21cnbao@gmail.com>
To: aubrey.li@linux.intel.com
Cc: linux-kernel@vger.kernel.org, mgorman@techsingularity.net,
	mingo@kernel.org, peterz@infradead.org,
	song.bao.hua@hisilicon.com, valentin.schneider@arm.com,
	vincent.guittot@linaro.org, yangyicong@huawei.com
Subject: Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup
Date: Fri, 17 Sep 2021 11:44:00 +0800	[thread overview]
Message-ID: <20210917034400.7620-1-21cnbao@gmail.com> (raw)
In-Reply-To: <125eb98a-241b-078f-1844-b0521425ed1e@linux.intel.com>

[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

  reply	other threads:[~2021-09-17  3:44 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-26 10:22 [RFC PATCH 0/9] Modify and/or delete SIS_PROP Mel Gorman
2021-07-26 10:22 ` [PATCH 1/9] sched/fair: Track efficiency of select_idle_sibling Mel Gorman
2021-07-26 10:22 ` [PATCH 2/9] sched/fair: Track efficiency of task recent_used_cpu Mel Gorman
2021-07-26 10:22 ` [PATCH 3/9] sched/fair: Track efficiency of select_idle_core Mel Gorman
2021-07-26 10:22 ` [PATCH 4/9] sched/fair: Use prev instead of new target as recent_used_cpu Mel Gorman
2021-07-26 10:22 ` [PATCH 5/9] sched/fair: Avoid a second scan of target in select_idle_cpu Mel Gorman
2021-07-26 10:22 ` [PATCH 6/9] sched/fair: Make select_idle_cpu() proportional to cores Mel Gorman
2021-07-26 10:22 ` [PATCH 7/9] sched/fair: Enforce proportional scan limits when scanning for an idle core Mel Gorman
2021-08-02 10:52   ` Song Bao Hua (Barry Song)
2021-08-04 10:22     ` Mel Gorman
2021-07-26 10:22 ` [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup Mel Gorman
2021-08-02 10:41   ` Song Bao Hua (Barry Song)
2021-08-04 10:26     ` Mel Gorman
2021-08-05  0:23       ` Aubrey Li
2021-09-17  3:44         ` Barry Song [this message]
2021-09-17  4:15         ` Barry Song
2021-09-17  9:11           ` Aubrey Li
2021-09-17 13:35             ` Mel Gorman
2021-07-26 10:22 ` [PATCH 9/9] sched/core: Delete SIS_PROP and rely on the idle cpu mask Mel Gorman

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