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

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Message-ID: <a21dc7a7-a8ce-4cbc-a393-1b2b5b8e75a1@nvidia.com>
Date: Tue, 18 Mar 2025 11:07:13 +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 11:48 PM, Tejun Heo wrote:
> On Mon, Mar 17, 2025 at 11:39:32PM +0100, Joel Fernandes wrote:
>> On 3/17/2025 11:16 PM, Tejun Heo wrote:
>>> 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,
> I'm not very familiar with DL but it looks like a stopped DL server would
> still be reserving bandwidth which limits what other actual DL users would
> be able to reserve without causing overflow. It looks like EXT's activation
> modes should be calling into dl_bw_manage() so that FAIR's and EXT's
> reservations can be retracted when not in use.

Ah, you raise a good point. Sorry, you were on to something and that makes sense
to me. Let me see how to wire it up. Basically, when we switch to full-mode from
say partial, we could/should remove the bandwidth reservation of the servers. I
think I confused the concept of "server not running" to "server reserving
bandwidth". My bad!

thanks,

 - Joel