linux-kernel - Re: [PATCH RFC 3/8] sched/ext: Add a DL server for sched

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Message-ID: <8d9e6300-113e-4a89-8fc3-2ae895c77794@nvidia.com>
Date: Mon, 17 Mar 2025 22:48:16 +0100
From: Joel Fernandes <joelagnelf@...dia.com>
To: Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org, Andrea Righi <arighi@...dia.com>,
 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>, Valentin Schneider <vschneid@...hat.com>,
 David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
 Luigi De Matteis <ldematteis123@...il.com>, paulmck@...nel.org,
 boqun.feng@...il.com
Subject: Re: [PATCH RFC 3/8] sched/ext: Add a DL server for sched_ext tasks

Hello, Peter, Tejun,

On 3/17/2025 6:06 PM, Peter Zijlstra wrote:
> On Mon, Mar 17, 2025 at 06:57:19AM -1000, Tejun Heo wrote:
>> Hello,
>>
>> On Mon, Mar 17, 2025 at 11:31:01AM +0100, Peter Zijlstra wrote:
>>> On Sat, Mar 15, 2025 at 07:15:27PM -0400, Joel Fernandes wrote:
>>>> If so, that will not handle the case where the system has both
>>>> FAIR and EXT tasks in the mix (EXT has a partial mode where certain tasks can be
>>>> made EXT with certain others left as FAIR) and FAIR runs 100% and starves EXT.
>>>
>>> Well, you did not mention that issue, you only babbled about RT.

You are right, I will add more details about this to the change log.

>>>
>>> I did point out that issue with ext, and TJ said this mixed mode wasn't
>>> really meant to be used or somesuch.
>>
>> It's true that most of the current use cases don't use mixed mode. That
>> said, some folks are interested in it and if we can prevent starvation from
>> fair saturating CPUs in mixed mode with a DL server, that'd be really nice.
>> Would it be possible to toggle the reservations depending on the ext's
>> operation mode?
> 
> Yeah, that should be doable.

Just to clarify, Tejun is suggesting that in mixed mode, we boost EXT
independent of FAIR. And in normal mode, we we boost both FAIR+EXT, because well
- nothing would be running as fair anyway.

But what is the point of doing that, if we have boost EXT independent of FAIR
anyway? We need that code _anyway_ due to mixed mode so it would not simplify
anything.

Or did Tejun mean something else about "toggle the reservations"?

thanks,

 - Joel