linux-kernel - Re: [RFT][PATCH v1 7/8] cpufreq: intel_pstate: Align perf domains with L2 cache

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Message-ID: <c8ea80f0-66ee-4c13-ae52-eefab161faf4@arm.com>
Date: Thu, 1 May 2025 14:30:32 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>,
 Linux PM <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 Lukasz Luba <lukasz.luba@....com>, Peter Zijlstra <peterz@...radead.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 Morten Rasmussen <morten.rasmussen@....com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
 Pierre Gondois <pierre.gondois@....com>,
 Christian Loehle <christian.loehle@....com>,
 Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFT][PATCH v1 7/8] cpufreq: intel_pstate: Align perf domains
 with L2 cache

On 30/04/2025 21:29, Rafael J. Wysocki wrote:
> On Sun, Apr 27, 2025 at 6:23 PM Dietmar Eggemann
> <dietmar.eggemann@....com> wrote:
>>
>> On 16/04/2025 20:10, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

[...]

>> There seems to be an issue with late CPU-hotplug-in and this alignment
>> on L2 cache boundaries.

[...]

> I see.
> 
> What happens is that cpu_cacheinfo hides information on offline CPUs,
> so when CPU20 goes online, it doesn't see any other CPUs sharing the
> L2 with it.  Accordingly, a PD is created just for itself.
> 
> When CPU21 goes online, it sees that CPU20 shares the L2 with it, so
> the code attempts to create a PD for them both which fails.
> 
> This could be addressed, but the code would need to be a bit more
> complex and the current hardware seems to do better with a PD per CPU,
> so I'll drop the $subject patch for now.

Ah OK, thanks!