linux-kernel - Re: [PATCH] sched: topology: make cache topology separate from cpu topology

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Message-ID: <Yisxg3izMrGVahOQ@hirez.programming.kicks-ass.net>
Date:   Fri, 11 Mar 2022 12:24:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qing Wang <wangqing@...o.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: topology: make cache topology separate from cpu
 topology

On Thu, Mar 10, 2022 at 04:58:44AM -0800, Qing Wang wrote:
> From: Wang Qing <wangqing@...o.com>
> 
> Some architectures(e.g. ARM64), caches are implemented below:
> cluster:              ****** cluster 0 *****      ****** cluster 1 *****
> core:                 0      1      2      3      4      5      6      7
> cache(Leveln):        **cache0**   **cache1**	 **cache2**   **cache3**
> sd_llc_id(current):   0      0      0      0      4      4      4      4
> sd_llc_id(should be): 0      0      2      2      4      4      6      6
> 
> Caches and cpus have different topology, this causes cpus_share_cache()
> return the wrong value, which will affect the CPU load balance.
> 
> Cache topology should be separated with CPU topology, it can be obtained
> from "next-level-cache" in DTS preferentially.

If your clusters do not have cache, then you're currently setting
SD_SHARE_PKG_RESOURCES wrong, if they do, things are correct.

If you want to represent L2, use the new fangled cluster level or
something, that's what it's there for.

That is, you can represent the above like:

	DIE:	0-7
	MC:	0-3,		4-7
	CLS:	0-1,1-2,	4-5,6-7

But if there is cache at MC, LLC is what it is.