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

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Date:   Fri, 2 Jun 2023 13:23:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Gautham R. Shenoy" <gautham.shenoy@....com>
Cc:     K Prateek Nayak <kprateek.nayak@....com>,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, x86@...nel.org
Subject: Re: [tip: sched/core] sched/fair: Multi-LLC select_idle_sibling()

On Fri, Jun 02, 2023 at 02:36:37PM +0530, Gautham R. Shenoy wrote:

> Yes, this is what the topology looks like
> 
> |---------------------------------------------------------------------------------| 
> |                                                                                 |
> |   ----------- ----------- -----------     ----------- ----------- -----------   |
> |   |(0-7)    | |(8-15)   | |(16-23)  |     |(48-55)  | |(56-63)  | |(64-71)  |   |
> |   | LLC0    | | LLC1    | | LLC2    |     | LLC6    | | LLC7    | | LLC8    |   |
> |   |(96-103) | |(104-111)| |(112-119)|     |(144-151)| |(152-159)| |(160-167)|   |
> |   ----------- ----------- -----------     ----------- ----------- -----------   |
> |                                                                                 |
> |                                                                                 |
> |   ----------- ----------- -----------     ----------- ----------- -----------   |
> |   |(24-31)  | |(32-39)  | |(40-47)  |     |(72-79)  | |(80-87)  | |(88-95)  |   |
> |   | LLC3    | | LLC4    | | LLC5    |     | LLC9    | | LLC10   | | LLC11   |   |
> |   |(120-127)| |(128-135)| |(136-143)|     |(168-175)| |(176-183)| |(184-191)|   |
> |   ----------- ----------- -----------     ----------- ----------- -----------   |
> |                                                                                 |
> |---------------------------------------------------------------------------------|

Yup, that's the pictures I found online.

> > I would think it is the latter since NPS4 does that but let me go verify.
> 
> 2 groups of 6 each is the vertical split which is NPS2.
> 
> 4 groups of 3 each is the vertical and horizontal split, which is
> NPS4.
> 
> In both these cases, currently the domain hierarchy
> 
> SMT --> MC --> NODE --> NUMA
> 
> where the NODE will be the parent of MC and be the 2nd level wakeup domain.
> 
> If we define CLS to be the group with 3 LLCs, which becomes the parent
> of the MC domain, then, the hierarchy would be
> 
> NPS1 : SMT --> MC --> CLS --> DIE
> NPS2 : SMT --> MC --> CLS --> NODE --> NUMA
> NPS4 : SMT --> MC --> CLS --> NUMA
> 
> NPS2 will have 5 domains within a single socket. Oh well!

I think cluster/CLS is taken for L2, we should not re-purpose that for
groups of L3.

Anyway, yes, 5 levels. Shouldn't be a problem though, right?