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

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Message-ID: <20160329201145.GC3430@twins.programming.kicks-ass.net>
Date:	Tue, 29 Mar 2016 22:11:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.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, Mar 29, 2016 at 01:10:56PM -0400, Steven Rostedt wrote:
> On Tue, 29 Mar 2016 18:04:01 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:

> > 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.

No, it should really stay one tracepoint, useful for all scheduling.

> > 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.

Why the complexity? Why not just tack those 32 bytes on and get on with
life?