linux-kernel - Re: [PATCH] sched: fair: don't depend on wake_wide if waker and wakee are already in same LLC

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Date:   Tue, 1 Jun 2021 08:59:18 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "valentin.schneider@....com" <valentin.schneider@....com>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "bristot@...hat.com" <bristot@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "guodong.xu@...aro.org" <guodong.xu@...aro.org>,
        yangyicong <yangyicong@...wei.com>,
        tangchengchang <tangchengchang@...wei.com>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH] sched: fair: don't depend on wake_wide if waker and
 wakee are already in same LLC

On Mon, May 31, 2021 at 10:21:55PM +0000, Song Bao Hua (Barry Song) wrote:
> The benchmark of tbenchs is still positive:
> 
> tbench4
> 
>                            5.13-rc4               5.13-rc4
>                                      disable-llc-wakewide/
> 
> Hmean     1       514.87 (   0.00%)      505.17 *  -1.88%*
> Hmean     2       914.45 (   0.00%)      918.45 *   0.44%*
> Hmean     4      1483.81 (   0.00%)     1485.38 *   0.11%*
> Hmean     8      2211.62 (   0.00%)     2236.02 *   1.10%*
> Hmean     16     2129.80 (   0.00%)     2450.81 *  15.07%*
> Hmean     32     5098.35 (   0.00%)     5085.20 *  -0.26%*
> Hmean     64     4797.62 (   0.00%)     4801.34 *   0.08%*
> Hmean     80     4802.89 (   0.00%)     4780.40 *  -0.47%*
> 
> I guess something which work across several LLC domains
> cause performance regression.
> 
> I wonder how your test will be like if you pin the testing
> to CPUs within one LLC?
> 

While I could do this, what would be the benefit? Running within one LLC
would be running the test in one small fraction of the entire machine as
the machine has multiple LLCs per NUMA node. A patch dealing with how the
scheduler works with respect to LLC should take different configurations
into consideration as best as possible.

-- 
Mel Gorman
SUSE Labs