linux-kernel - Re: [PATCH v2 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

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Date:   Mon, 7 Feb 2022 20:44:22 +0530
From:   "Gautham R. Shenoy" <gautham.shenoy@....com>
To:     Barry Song <21cnbao@...il.com>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Yicong Yang <yangyicong@...ilicon.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        prime.zeng@...wei.com,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        ego@...ux.vnet.ibm.com, Linuxarm <linuxarm@...wei.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Guodong Xu <guodong.xu@...aro.org>
Subject: Re: [PATCH v2 2/2] sched/fair: Scan cluster before scanning LLC in
 wake-up path


On Fri, Feb 04, 2022 at 11:28:25PM +1300, Barry Song wrote:

> > We already figured out that there are no idle CPUs in this cluster. So dont
> > we gain performance by picking a idle CPU/core in the neighbouring cluster.
> > If there are no idle CPU/core in the neighbouring cluster, then it does make
> > sense to fallback on the current cluster.
> 
> What you suggested is exactly the approach we have tried at the first beginning
> during debugging. but we didn't gain performance according to benchmark, we
> were actually losing. that is why we added this line to stop ping-pong:
>          /* Don't ping-pong tasks in and out cluster frequently */
>          if (cpus_share_resources(target, prev_cpu))
>             return target;
> 
> If we delete this, we are seeing a big loss of tbench while system
> load is medium
> and above.

Thanks for clarifying this Barry. Indeed, if the workload is sensitive
to data ping-ponging across L2 clusters, this heuristic makes sense. I
was thinking of workloads that require lower tail latency, in which
case exploring the larger LLC would have made more sense, assuming
that the larger LLC has an idle core/CPU.

In the absence of any hints from the workload, like something that
Peter had previous suggested
(https://lore.kernel.org/lkml/YVwnsrZWrnWHaoqN@hirez.programming.kicks-ass.net/),
optimizing for cache-access seems to be the right thing to do.


> 
> Thanks
> Barry

--
Thanks and Regards
gautham.