linux-kernel - Re: [tip: sched/core] sched/fair: Multi-LLC select_idle

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Date:   Thu, 1 Jun 2023 13:56:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     K Prateek Nayak <kprateek.nayak@....com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, x86@...nel.org,
        Gautham Shenoy <gautham.shenoy@....com>
Subject: Re: [tip: sched/core] sched/fair: Multi-LLC select_idle_sibling()

On Thu, Jun 01, 2023 at 01:13:26PM +0200, Peter Zijlstra wrote:
> 
> This DeathStarBench thing seems to suggest that scanning up to 4 CCDs
> isn't too much of a bother; so perhaps something like so?
> 
> (on top of tip/sched/core from just a few hours ago, as I had to 'fix'
> this patch and force pushed the thing)
> 
> And yeah, random hacks and heuristics here :/ Does there happen to be
> additional topology that could aid us here? Does the CCD fabric itself
> have a distance metric we can use?

  https://www.anandtech.com/show/16529/amd-epyc-milan-review/4

Specifically:

  https://images.anandtech.com/doci/16529/Bounce-7763.png

That seems to suggest there are some very minor distance effects in the
CCD fabric. I didn't read the article too closely, but you'll note that
the first 4 CCDs have inter-CCD latency < 100 while the rest has > 100.

Could you also test on a Zen2 Epyc, does that require nr=8 instead of 4?
Should we perhaps write it like: 32 / llc_size ?

The Zen2 picture:

  https://images.anandtech.com/doci/16315/Bounce-7742.png

Shows a more pronounced CCD fabric topology, you can really see the 2
CCX inside the CCD but also there's two ligher green squares around the
CCDs themselves.