linux-kernel - Re: [RFC PATCH 6/8] cpufreq: intel

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Message-ID: <fa623b5e-721a-47fd-84c8-1088d9a6a24a@arm.com>
Date: Tue, 1 Oct 2024 10:57:03 +0100
From: Christian Loehle <christian.loehle@....com>
To: srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 peterz@...radead.org, juri.lelli@...hat.com, mingo@...hat.com,
 dietmar.eggemann@....com, vschneid@...hat.com, vincent.guittot@...aro.org,
 Johannes.Thumshirn@....com, adrian.hunter@...el.com, ulf.hansson@...aro.org,
 bvanassche@....org, andres@...razel.de, asml.silence@...il.com,
 linux-block@...r.kernel.org, io-uring@...r.kernel.org, qyousef@...alina.io,
 dsmythies@...us.net, axboe@...nel.dk
Subject: Re: [RFC PATCH 6/8] cpufreq: intel_pstate: Remove iowait boost

On 9/30/24 21:35, srinivas pandruvada wrote:
> On Mon, 2024-09-30 at 20:03 +0200, Rafael J. Wysocki wrote:
>> +Srinivas who can say more about the reasons why iowait boosting
>> makes
>> a difference for intel_pstate than I do.
>>

Hi Srinivas,

> It makes difference on Xeons and also GFX performance.

AFAIU the GFX performance with iowait boost is a regression though,
because it cuts into the system power budget (CPU+GPU), especially
on desktop and mobile chips (but also some servers), no?
https://lore.kernel.org/lkml/20180730220029.81983-1-srinivas.pandruvada@linux.intel.com/
https://lore.kernel.org/lkml/e7388bf4-deb1-34b6-97d7-89ced8e78ef1@intel.com/
Or is there a reported case where iowait boosting helps
graphics workloads?

> The actual gains will be model specific as it will be dependent on
> hardware algorithms and EPP.
> 
> It was introduced to solve regression in Skylake xeons. But even in the
> recent servers there are gains.
> Refer to
> https://lkml.iu.edu/hypermail/linux/kernel/1806.0/03574.html

Did you look into PELT utilization values at that time?
I see why intel_pstate might be worse off than schedutil wrt removing
iowait boosting and do see two remedies essentially:
1. Boost after all sleeps (less aggressively), although I'm not a huge fan of
this.
2. If the gap between util_est and HWP-determined frequency is too large
then apply some boost. A sort of fallback on a schedutil strategy.
That would of course require util_est to be significantly large in those
scenarios.

I might try to propose something for 2, although as you can probably
guess, playing with HWP is somewhat uncharted waters for me.

Since intel_pstate will actually boost into unsustainable P-states,
there should be workloads that regress with iowait boosting. I'll
go looking for those.