linux-kernel - Re: [PATCH 1/2] sched/fair: Use weight of SD_NUMA domain in find_busiest

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Date:   Tue, 21 Dec 2021 14:05:59 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <Valentin.Schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Mike Galbraith <efault@....de>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Gautham Shenoy <gautham.shenoy@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/fair: Use weight of SD_NUMA domain in find_busiest_group

On Tue, 21 Dec 2021 at 12:32, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Tue, Dec 21, 2021 at 11:53:50AM +0100, Vincent Guittot wrote:
> > On Fri, 10 Dec 2021 at 10:33, Mel Gorman <mgorman@...hsingularity.net> wrote:
> > >
> > > find_busiest_group uses the child domain's group weight instead of
> > > the sched_domain's weight that has SD_NUMA set when calculating the
> > > allowed imbalance between NUMA nodes. This is wrong and inconsistent
> > > with find_idlest_group.
> >
> > I agree that find_busiest_group and find_idlest_group should be
> > consistent and use the same parameters but I wonder if sched_domain's
> > weight is the right one to use instead of the target group's weight.
> >
>
> Ok
>
> > IIRC, the goal of adjust_numa_imbalance is to keep some threads on the
> > same node as long as we consider that there is no performance impact
> > because of sharing  resources as they can even take advantage of
> > locality if they interact.
>
> Yes.
>
> > So we consider that tasks will not be
> > impacted by sharing resources if they use less than 25% of the CPUs of
> > a node. If we use the sd->span_weight instead, we consider that we can
> > pack threads in the same node as long as it uses less than 25% of the
> > CPUs in all nodes.
> >
>
> I assume you mean the target group weight instead of the node. The

I wanted to say that with this patch, we consider the imbalance
acceptable if the number of threads in a node is less than 25% of all
CPUs of all nodes (for this numa level) , but 25% of all CPUs of all
nodes can be more that the number of CPUs in the group.

So I would have changed find_idlest_group instead of changing find_busiest_group

> primary resource we are concerned with is memory bandwidth and it's a
> guess because we do not know for sure where memory channels are or how
> they are configured in this context and it may or may not be correlated
> with groups. I think using the group instead would deserve a series on
> its own after settling on an imbalance number when there are multiple
> LLCs per node.

I haven't look yet at the patch2 for multiple LLC per node

>
> --
> Mel Gorman
> SUSE Labs