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

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Message-ID: <bf96be8a-2358-b9ab-b8eb-d0b8b94ed0d7@arm.com>
Date:   Wed, 30 Oct 2019 17:35:55 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        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>,
        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 30/10/2019 17:24, Dietmar Eggemann wrote:
> 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.
> 

Well from the code nobody but us (asymmetric capacity systems) set
SD_BALANCE_WAKE. I was however curious if there were some folks who set it
with out of tree code for some reason.

As Dietmar said, not having SD_BALANCE_WAKE means you'll never go through
the slow path on wakeups, because there is no domain with SD_BALANCE_WAKE for
the domain loop to find. Depending on your topology you most likely will
go through it on fork or exec though.

IOW wake_wide() is not really widening the wakeup scan on wakeups using
mainline topology code (disregarding asymmetric capacity systems), which
sounds a bit... off.