linux-kernel - Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask in sched domain

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Message-ID: <3f1571ea-b74c-fc40-2696-39ef3fe8b968@linux.intel.com>
Date:   Sat, 12 Sep 2020 07:04:29 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Qais Yousef <qais.yousef@....com>, Aubrey Li <aubrey.li@...el.com>
Cc:     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,
        valentin.schneider@....com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask
 in sched domain

On 2020/9/12 0:28, Qais Yousef wrote:
> On 09/10/20 13:42, Aubrey Li wrote:
>> Added idle cpumask to track idle cpus in sched domain. When a CPU
>> enters idle, its corresponding bit in the idle cpumask will be set,
>> and when the CPU exits idle, its bit will be cleared.
>>
>> When a task wakes up to select an idle cpu, scanning idle cpumask
>> has low cost than scanning all the cpus in last level cache domain,
>> especially when the system is heavily loaded.
>>
>> Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>
>> ---
>>  include/linux/sched/topology.h | 13 +++++++++++++
>>  kernel/sched/fair.c            |  4 +++-
>>  kernel/sched/topology.c        |  2 +-
>>  3 files changed, 17 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
>> index fb11091129b3..43a641d26154 100644
>> --- a/include/linux/sched/topology.h
>> +++ b/include/linux/sched/topology.h
>> @@ -65,8 +65,21 @@ struct sched_domain_shared {
>>  	atomic_t	ref;
>>  	atomic_t	nr_busy_cpus;
>>  	int		has_idle_cores;
>> +	/*
>> +	 * Span of all idle CPUs in this domain.
>> +	 *
>> +	 * NOTE: this field is variable length. (Allocated dynamically
>> +	 * by attaching extra space to the end of the structure,
>> +	 * depending on how many CPUs the kernel has booted up with)
>> +	 */
>> +	unsigned long	idle_cpus_span[];
> 
> Can't you use cpumask_var_t and zalloc_cpumask_var() instead?

I can use the existing free code. Do we have a problem of this?

> 
> The patch looks useful. Did it help you with any particular workload? It'd be
> good to expand on that in the commit message.
> 
Odd, that included in patch v1 0/1, did you receive it?

Thanks,
-Aubrey