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

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Message-ID: <Z9imufPw3NbcpqJV@slm.duckdns.org>
Date: Mon, 17 Mar 2025 12:48:25 -1000
From: Tejun Heo <tj@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
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 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.

Thanks.

-- 
tejun