linux-kernel - Re: [PATCH v3] sched/fair: filter out overloaded cpus in SIS

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Date:   Fri, 20 May 2022 10:48:38 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Josh Don <joshdon@...gle.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/fair: filter out overloaded cpus in SIS


On 5/20/22 6:16 AM, Tim Chen Wrote:
> On Thu, 2022-05-05 at 20:23 +0800, Abel Wu wrote:
>>
>> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
>> ---
>>   include/linux/sched/topology.h | 12 ++++++++++
>>   kernel/sched/core.c            | 38 ++++++++++++++++++++++++++++++
>>   kernel/sched/fair.c            | 43 +++++++++++++++++++++++++++-------
>>   kernel/sched/idle.c            |  1 +
>>   kernel/sched/sched.h           |  4 ++++
>>   kernel/sched/topology.c        |  4 +++-
>>   6 files changed, 92 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
>> index 56cffe42abbc..95c7ad1e05b5 100644
>> --- a/include/linux/sched/topology.h
>> +++ b/include/linux/sched/topology.h
>> @@ -81,8 +81,20 @@ struct sched_domain_shared {
>>   	atomic_t	ref;
>>   	atomic_t	nr_busy_cpus;
>>   	int		has_idle_cores;
>> +
>> +	/*
>> +	 * Tracking of the overloaded cpus can be heavy, so start
>> +	 * a new cacheline to avoid false sharing.
>> +	 */
>> +	atomic_t	nr_overloaded_cpus ____cacheline_aligned;
> 
> Abel,
> 
> This is nice work. I have one comment.
> 
> The update and reading of nr_overloaded_cpus will incur cache bouncing cost.
> As far as I can tell, this counter is used to determine if we should bail out
> from the search for an idle CPUs if the system is heavily loaded.  And I
> hope we can avoid using atomic counter in these heavily used scheduler paths.
> The logic to filter overloaded CPUs only need overloaded_cpus[]
> mask and not the nr_overloaded_cpus counter.
> 
> So I recommend that you break out the logic of using nr_overloaded_cpus
> atomic counter to detect LLC has heavy load as a second patch so that
> it can be evaluated on its own merit.
> 
> That functionality overlaps with Chen Yu's patch to limit search depending
> on load, so it will be easier to compare the two approaches if it is separated.
> 
> Otherwise, the logic in the patch to use overloaded_cpus[]
> mask to filter out the overloaded cpus looks fine and complements
> Chen Yu's patch.

OK, will do. And ideally the nr_overloaded_cpus atomic can be replaced
by Chen's patch.

Thanks & BR
Abel