linux-kernel - Re: [RFC PATCH 14/17] sched: Add deadline tracepoints

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Message-ID: <aKSD3Pm_9qqmEaNv@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 19 Aug 2025 16:02:04 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, linux-trace-kernel@...r.kernel.org,
	Nam Cao <namcao@...utronix.de>, Tomas Glozar <tglozar@...hat.com>,
	Juri Lelli <jlelli@...hat.com>,
	Clark Williams <williams@...hat.com>,
	John Kacur <jkacur@...hat.com>
Subject: Re: [RFC PATCH 14/17] sched: Add deadline tracepoints

On 19/08/25 12:34, Gabriele Monaco wrote:
> 
> 
> On Tue, 2025-08-19 at 12:12 +0200, Peter Zijlstra wrote:
> > On Tue, Aug 19, 2025 at 11:56:57AM +0200, Juri Lelli wrote:
> > > Hi!
> > > 
> > > On 14/08/25 17:08, Gabriele Monaco wrote:

...

> > > > @@ -1482,6 +1486,7 @@ static void update_curr_dl_se(struct rq
> > > > *rq, struct sched_dl_entity *dl_se, s64
> > > >  
> > > >  throttle:
> > > >  	if (dl_runtime_exceeded(dl_se) || dl_se->dl_yielded) {
> > > > +		trace_sched_dl_throttle_tp(dl_se);
> > > >  		dl_se->dl_throttled = 1;
> > > 
> > > I believe we also need to trace the dl_check_constrained_dl()
> > > throttle, please take a look.
> 
> Probably yes, strangely I couldn't see failures without it, but it may
> be down to my test setup. I'm going to have a look.

Not sure if you tested with constrained (deadline != period) tasks.

> > > Also - we discussed this point a little already offline - but I
> > > still wonder if we have to do anything special for dl-server defer.
> > > Those entities are started as throttled until 0-lag, so maybe we
> > > should still trace them explicitly as so?
> 
> The naming might need a bit of a consistency check here, but for the
> monitor, the server is running, armed or preempted. Before the 0-lag,
> it will never be running, so it will stay as armed (fair tasks running)
> or preempted (rt tasks running).
> 
> armed and preempted have the _throttled version just to indicate an
> explicit throttle event occurred.
> 
> We might want to start it in the armed_throttled if we are really
> guaranteed not to see a throttle event, but I think that would
> complicate the model considerably.
> 
> We could instead validate the 0-lag concept in a separate, server-
> specific model.
> 
> Does this initial model feel particularly wrong for the server case?

No it doesn't atm. :-) Thanks for the additional information.

> > > In addition, since it's related, maybe we should do something about
> > > sched_switch event, that is currently not aware of deadlines,
> > > runtimes, etc.
> 
> I'm not sure I follow you here, what relation with switch and
> runtime/deadline should we enforce?
> 
> We don't really force the switch to occur timely after throttling, is
> that what you mean?
> Or a switch must occur again timely after replenishing?

Hummm, yeah I was wondering if we need something along these lines, but
we can also maybe leave it for the future.

> > As per the whole _tp() thing, you can attach to the actual
> > sched_switch tracepoint with a module and read whatever you want.
> 
> Yeah I believe Juri referred to model constraints on the already
> existing events rather than new tracepoints here.

Separately from this series, maybe we should put such a module/bpf thing
somewhere shared, so it's easier to use it when needed.