linux-kernel - Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier for hotplug

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Message-ID: <611a1604-b920-4963-8a1c-7e52bf8c81e5@arm.com>
Date: Thu, 13 Feb 2025 14:57:32 +0000
From: Christian Loehle <christian.loehle@....com>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
 Jon Hunter <jonathanh@...dia.com>, Thierry Reding <treding@...dia.com>,
 Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutny <mkoutny@...e.com>,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Phil Auld <pauld@...hat.com>, Qais Yousef <qyousef@...alina.io>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 "Joel Fernandes (Google)" <joel@...lfernandes.org>,
 Suleiman Souhlal <suleiman@...gle.com>, Aashish Sharma <shraash@...gle.com>,
 Shin Kawamura <kawasin@...gle.com>,
 Vineeth Remanan Pillai <vineeth@...byteword.org>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier
 for hotplug

On 2/13/25 14:51, Juri Lelli wrote:
> On 13/02/25 13:38, Christian Loehle wrote:
>> On 2/13/25 13:33, Juri Lelli wrote:
> 
> ...
> 
>>> Not sure I get what your worry is, sorry. In my understanding when the
>>> last cpu of a policy/cluster gets offlined the corresponding sugov
>>> kthread gets stopped as well (sugov_exit)?
>>>
>>
>> The other way round.
>> We may have sugov kthread of cluster [6,7] affined to CPU1. Is it
>> guaranteed that we cannot offline CPU1 (while CPU6 or CPU7 are still
>> online)?
> 
> Uhu, is this a sane/desired setup? Anyway, I would say that if CPU1 is
> offlined sugov[6,7] will need to be migrated someplace else.

Sane? I guess that's to be discussed. It is definitely desirable
unfortunately.
As mentioned I experimented with having sugov DL tasks (as they cause
a lot of idle wakeups (which are expensive on the bigger CPUs)) both
always run locally and never IPI (but that means we have contention and
still run a double switch on an 'expensive' CPU) and run that on a little
CPU and the latter had much better results.

> 
>> Or without the affinity:
>> cluster [6,7] with isolcpu=6 (i.e. sugov kthread of that cluster can
>> only run on CPU7). Is offlining of CPU6 then prevented (as long as
>> CPU7 is online)?
>> I don't see how.
>> Anyway we probably want to change isolcpu and affinity to merely be 
>> a suggestion for the sugov DL case. Fundamentally it belongs to what
>> is run on that CPU anyway.
> 
> I would tend to agree.

I'll write something up.