linux-kernel - Re: [PATCH V2 3/3] sched/deadline: Tracepoints for deadline scheduler

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Date:	Tue, 29 Mar 2016 13:10:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Juri Lelli <juri.lelli@....com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH V2 3/3] sched/deadline: Tracepoints for deadline
 scheduler

On Tue, 29 Mar 2016 18:04:01 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Mar 29, 2016 at 11:57:00AM -0400, Steven Rostedt wrote:
> > Hmm, I probably could add tracing infrastructure that would let us
> > extend existing tracepoints. That is, without modifying sched_switch,
> > we could add a new tracepoint that when enabled, would attach itself to
> > the sched_switch tracepoint and record different information. Like a
> > special sched_switch_deadline tracepoint, that would record the existing
> > runtime,deadline and period for deadline tasks. It wont add more
> > tracepoints into the core scheduler, but use the existing one.  
> 
> Urgh; maybe. But I would would not want the new thing to be called
> _deadline, maybe _v{n} id anything and have a KERN_WARNING emitted when
> people enable the old one.

I wasn't thinking of having a new sched switch, I was thinking of
having multiple ones. And not versions, as the one for a deadline task
wouldn't be applicable for a non deadline task. But regardless, I'm
also thinking of something else.

> 
> Ideally we'd rename the old one, but I suspect even that would break
> stuff :/

Yes, we don't want to get rid of the old one. But it shouldn't break
anything if we extend it. I'm thinking of extending it with a dynamic
array to store the deadline task values (runtime, period). And for non
deadline tasks, the array would be empty (size zero). I think that
could be doable and maintain backward compatibility.

-- Steve