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

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Message-ID: <4b8a25f7-c7c8-467f-8d03-6e590712ffbd@nvidia.com>
Date: Mon, 17 Mar 2025 23:39:32 +0100
From: Joel Fernandes <joelagnelf@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, 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



On 3/17/2025 11:16 PM, Tejun Heo wrote:
> Hello,
> 
> On Mon, Mar 17, 2025 at 10:48:16PM +0100, Joel Fernandes wrote:
> ...
>> 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"?
> 
> My understanding is that if we have both FAIR and EXT's DL servers reserving
> execution time all the time, we'd be reserving execution time for something
> which can't be active, so the only change necessary I think is just
> retracting FAIR's or EXT's reservation whent we know they are not active
> (ie. if EXT is not loaded or EXT is loaded in full-sys mode).
> 
Ah, I see what you mean. We already have a 'toggle' like that though because if
FAIR or EXT is not running (due to whatever reason), we would have already
called 'dl_server_stop()' or would never have called 'dl_server_start()'.

On the other hand, even if full-sys-mode, we need the EXT server to boost it to
above RT if EXT is running, so we need its server initialized and ready to go.

Let me know if I missed anything though, thanks,

 - Joel