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

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Date:   Fri, 2 Jun 2023 10:47:07 +0530
From:   K Prateek Nayak <kprateek.nayak@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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()

Hello Peter,

On 6/1/2023 8:21 PM, Peter Zijlstra wrote:
> On Thu, Jun 01, 2023 at 02:00:01PM +0200, Peter Zijlstra wrote:
>> On Thu, Jun 01, 2023 at 01:56:43PM +0200, Peter Zijlstra wrote:
>>> 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.
>>
>> I can't seem to find pretty pictures for Zen4 Epyc; what does that want?
>> That's even bigger at 96/8=12 LLCs afaict.
> 
> Going by random pictures on the interweb again, it looks like this Zen4
> thing wants either 2 groups of 6 each, or 4 groups of 3.

I would think it is the latter since NPS4 does that but let me go verify.
Thank you for digging into this and providing the patch to extend the x86
topology in the thread :)

> 
> But you have the hardware, so I'll let you figure it out.

--
Thanks and Regards,
Prateek