linux-kernel - Re: [PATCH 00/10] sched/fair: wake

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Date:   Mon, 26 Apr 2021 16:09:40 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Parth Shah <parth@...ux.ibm.com>
Subject: Re: [PATCH 00/10] sched/fair: wake_affine improvements

* Mel Gorman <mgorman@...hsingularity.net> [2021-04-23 09:25:32]:

> On Thu, Apr 22, 2021 at 03:53:16PM +0530, Srikar Dronamraju wrote:
> > Recently we found that some of the benchmark numbers on Power10 were lesser
> > than expected. Some analysis showed that the problem lies in the fact that
> > L2-Cache on Power10 is at core level i.e only 4 threads share the L2-cache.
> > 
> 
> I didn't get the chance to review this properly although I am suspicious
> of tracking idle_core and updating that more frequently. It becomes a very
> hot cache line that bounces. I did experiement with tracking an idle core
> but the data either went stale too quickly or the updates incurred more
> overhead than a reduced search saved.
> 

This change does increase the number of times we read the idle-core.  There
are also more places where we try to update the idle-core. However I feel
the number of times, we actually update the idle-core now will be much
lesser than previous, because we are mostly doing a conditional update. i.e
we are updating the idle-core only if the waking up CPU happens to be part
of our core.

Also if the system is mostly lightly loaded, we check for
available_idle_cpu, so we may not look for an idle-core. If the system is
running a CPU intensive task, then the idle-core will most likely to be -1.
Its only the cases where the system utilization keeps swinging between
lightly loaded to heavy load, that we would end up checking and setting
idle-core.

Do let me know your thoughts.

-- 
Thanks and Regards
Srikar Dronamraju