linux-kernel - Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance

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Message-ID: <564ca629-5c34-dbd1-8e64-2da6910b18a3@arm.com>
Date:   Wed, 30 Oct 2019 17:24:47 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Phil Auld <pauld@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Hillf Danton <hdanton@...a.com>,
        Parth Shah <parth@...ux.ibm.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance

On 30.10.19 15:39, Phil Auld wrote:
> Hi Vincent,
> 
> On Mon, Oct 28, 2019 at 02:03:15PM +0100 Vincent Guittot wrote:

[...]

>>> When you say slow versus fast wakeup paths what do you mean? I'm still
>>> learning my way around all this code.
>>
>> When task wakes up, we can decide to
>> - speedup the wakeup and shorten the list of cpus and compare only
>> prev_cpu vs this_cpu (in fact the group of cpu that share their
>> respective LLC). That's the fast wakeup path that is used most of the
>> time during a wakeup
>> - or start to find the idlest CPU of the system and scan all domains.
>> That's the slow path that is used for new tasks or when a task wakes
>> up a lot of other tasks at the same time

[...]

Is the latter related to wake_wide()? If yes, is the SD_BALANCE_WAKE
flag set on the sched domains on your machines? IMHO, otherwise those
wakeups are not forced into the slowpath (if (unlikely(sd))?

I had this discussion the other day with Valentin S. on #sched and we
were not sure how SD_BALANCE_WAKE is set on sched domains on
!SD_ASYM_CPUCAPACITY systems.