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Subject: Re: [RFC PATCH v1 1/1] sched/fair: select idle cpu from idle cpumask
 in sched domain
From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
To:     Qais Yousef <qais.yousef@arm.com>, Aubrey Li <aubrey.li@intel.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        valentin.schneider@arm.com, tim.c.chen@linux.intel.com,
        linux-kernel@vger.kernel.org
References: <20200910054203.525420-1-aubrey.li@intel.com>
 <20200910054203.525420-2-aubrey.li@intel.com>
 <20200911162853.xldy6fvvqph2lahj@e107158-lin.cambridge.arm.com>
 <3f1571ea-b74c-fc40-2696-39ef3fe8b968@linux.intel.com>
Message-ID: <8a853369-c8f0-b9e8-f4cb-481d73abefaf@linux.intel.com>
Date:   Sat, 12 Sep 2020 07:16:36 +0800
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On 2020/9/12 7:04, Li, Aubrey wrote:
> 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@linux.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?

I found it at here:

https://lkml.org/lkml/2020/9/11/645

> 
> Thanks,
> -Aubrey
>