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From:   Jirka Hladky <jhladky@redhat.com>
Date:   Wed, 13 May 2020 18:20:53 +0200
Message-ID: <CAE4VaGBq5+ucS4p+0AzFsNP7YDsg7dLZ73dzuhBerHiM4EYP_Q@mail.gmail.com>
Subject: Re: [PATCH 00/13] Reconcile NUMA balancing decisions with the load
 balancer v6
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Phil Auld <pauld@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Hillf Danton <hdanton@sina.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Douglas Shakshober <dshaks@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Joe Mario <jmario@redhat.com>, Bill Gray <bgray@redhat.com>
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Thank you, Mel!

I think I have to make sure we cover the scenario you have targeted
when developing adjust_numa_imbalance:

=======================================================================
https://github.com/torvalds/linux/blob/4f8a3cc1183c442daee6cc65360e3385021131e4/kernel/sched/fair.c#L8910

/*
* Allow a small imbalance based on a simple pair of communicating
* tasks that remain local when the source domain is almost idle.
*/
=======================================================================

Could you point me to a benchmark for this scenario? I have checked
https://github.com/gormanm/mmtests
and we use lots of the same benchmarks but I'm not sure if we cover
this particular scenario.

Jirka


On Wed, May 13, 2020 at 5:30 PM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Wed, May 13, 2020 at 04:57:15PM +0200, Jirka Hladky wrote:
> > Hi Mel,
> >
> > we have tried the kernel with adjust_numa_imbalance() crippled to just
> > return the imbalance it's given.
> >
> > It has solved all the performance problems I have reported.
> > Performance is the same as with 5.6 kernel (before the patch was
> > applied).
> >
> > * solved the performance drop upto 20%  with single instance
> > SPECjbb2005 benchmark on 8 NUMA node servers (particularly on AMD EPYC
> > Rome systems) => this performance drop was INCREASING with higher
> > threads counts (10% for 16 threads and 20 % for 32 threads)
> > * solved the performance drop for low load scenarios (SPECjvm2008 and NAS)
> >
> > Any suggestions on how to proceed? One approach is to turn
> > "imbalance_min" into the kernel tunable. Any other ideas?
> >
> > https://github.com/torvalds/linux/blob/4f8a3cc1183c442daee6cc65360e3385021131e4/kernel/sched/fair.c#L8914
> >
>
> Complete shot in the dark but restore adjust_numa_imbalance() and try
> this
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1a9983da4408..0b31f4468d5b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2393,7 +2393,7 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
>         struct rq_flags rf;
>
>  #if defined(CONFIG_SMP)
> -       if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
> +       if (sched_feat(TTWU_QUEUE)) {
>                 sched_clock_cpu(cpu); /* Sync clocks across CPUs */
>                 ttwu_queue_remote(p, cpu, wake_flags);
>                 return;
>
> --
> Mel Gorman
> SUSE Labs
>


-- 
-Jirka