linux-kernel - Re: [PATCH v2 0/8] sched/fair: rework the CFS load balance

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Date:   Fri, 30 Aug 2019 08:46:53 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     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>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2 0/8] sched/fair: rework the CFS load balance

Hi Phil,

On Thu, 29 Aug 2019 at 21:23, Phil Auld <pauld@...hat.com> wrote:
>
> On Thu, Aug 01, 2019 at 04:40:16PM +0200 Vincent Guittot wrote:
> > Several wrong task placement have been raised with the current load

> >
> > --
> > 2.7.4
> >
>
> I keep expecting a v3 so I have not dug into all the patches in detail. However, I've

v3 is under preparation

> been working with them from Vincent's tree while they were under development so I thought
> I'd add some results.

Yes. thanks for your help.

>
> The workload is a test our perf team came up with to illustrate the issues we were seeing
> with imbalance in the presence of group scheduling.
>
> On a 4-numa X 20 cpu system (smt on) we run a 76 thread lu.C benchmark from the NAS Parallel
> suite. And at the same time run 2 stress cpu burn processes.  The GROUP test puts the
> benchmark and the stress processes each in its own cgroup.  The NORMAL case puts them all
> in the first cgroup.  The results show first the average number of threads of each type
> running on each of the numa nodes based on sampling taken during the run.  This is followed
> by the lu.C benchmark results. There are 3 runs of GROUP and 2 runs of NORMAL shown.
>
> Before (linux-5.3-rc1+  @  a1dc0446d649)
>
> lu.C.x_76_GROUP_1.stress.ps.numa.hist   Average    0.00  1.00  1.00
> lu.C.x_76_GROUP_2.stress.ps.numa.hist   Average    0.00  1.00  1.00
> lu.C.x_76_GROUP_3.stress.ps.numa.hist   Average    0.00  1.00  1.00
> lu.C.x_76_NORMAL_1.stress.ps.numa.hist  Average    1.15  0.23  0.00  0.62
> lu.C.x_76_NORMAL_2.stress.ps.numa.hist  Average    1.67  0.00  0.00  0.33
>
> lu.C.x_76_GROUP_1.ps.numa.hist   Average    30.45  6.95  4.52  34.08
> lu.C.x_76_GROUP_2.ps.numa.hist   Average    32.33  8.94  9.21  25.52
> lu.C.x_76_GROUP_3.ps.numa.hist   Average    30.45  8.91  12.09  24.55
> lu.C.x_76_NORMAL_1.ps.numa.hist  Average    18.54  19.23  19.69  18.54
> lu.C.x_76_NORMAL_2.ps.numa.hist  Average    17.25  19.83  20.00  18.92
>
> ============76_GROUP========Mop/s===================================
> min     q1      median  q3      max
> 2119.92 2418.1  2716.28 3147.82 3579.36
> ============76_GROUP========time====================================
> min     q1      median  q3      max
> 569.65  660.155 750.66  856.245 961.83
> ============76_NORMAL========Mop/s===================================
> min     q1      median  q3      max
> 30424.5 31486.4 31486.4 31486.4 32548.4
> ============76_NORMAL========time====================================
> min     q1      median  q3      max
> 62.65   64.835  64.835  64.835  67.02
>
>
> After (linux-5.3-rc1+  @  a1dc0446d649 + this v2 series pulled from
> Vincent's git on ~8/15)
>
> lu.C.x_76_GROUP_1.stress.ps.numa.hist   Average    0.36  1.00  0.64
> lu.C.x_76_GROUP_2.stress.ps.numa.hist   Average    1.00  1.00
> lu.C.x_76_GROUP_3.stress.ps.numa.hist   Average    1.00  1.00
> lu.C.x_76_NORMAL_1.stress.ps.numa.hist  Average    0.23  0.15  0.31  1.31
> lu.C.x_76_NORMAL_2.stress.ps.numa.hist  Average    1.00  0.00  0.00  1.00
>
> lu.C.x_76_GROUP_1.ps.numa.hist   Average    18.91  18.36  18.91  19.82
> lu.C.x_76_GROUP_2.ps.numa.hist   Average    18.36  18.00  19.91  19.73
> lu.C.x_76_GROUP_3.ps.numa.hist   Average    18.17  18.42  19.25  20.17
> lu.C.x_76_NORMAL_1.ps.numa.hist  Average    19.08  20.00  18.62  18.31
> lu.C.x_76_NORMAL_2.ps.numa.hist  Average    18.09  19.91  19.18  18.82
>
> ============76_GROUP========Mop/s===================================
> min     q1      median  q3      max
> 32304.1 33176   34047.9 34166.8 34285.7
> ============76_GROUP========time====================================
> min     q1      median  q3      max
> 59.47   59.68   59.89   61.505  63.12
> ============76_NORMAL========Mop/s===================================
> min     q1      median  q3      max
> 29825.5 32454   32454   32454   35082.5
> ============76_NORMAL========time====================================
> min     q1      median  q3      max
> 58.12   63.24   63.24   63.24   68.36
>
>
> I had initially tracked this down to two issues. The first was picking the wrong
> group in find_busiest_group due to using the average load. The second was in
> fix_small_imbalance(). The "load" of the lu.C tasks was so low it often failed
> to move anything even when it did find a group that was overloaded (nr_running
> > width). I have two small patches which fix this but since Vincent was embarking
> on a re-work which also addressed this I dropped them.
>
> We've also run a series of performance tests we use to check for regressions and
> did not find any bad results on our workloads and systems.
>
> So...
>
> Tested-by: Phil Auld <pauld@...hat.com>

Thanks for testing

Vincent

>
>
> Cheers,
> Phil
> --