linux-kernel - Re: [PATCH RFC] sched: add notifier for process migration

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Date:	Wed, 14 Oct 2009 11:26:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, Avi Kivity <avi@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH RFC] sched: add notifier for process migration

On Wed, 2009-10-14 at 09:05 +0200, Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
> > @@ -1981,6 +1989,12 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> >  #endif
> >  		perf_swcounter_event(PERF_COUNT_SW_CPU_MIGRATIONS,
> >  				     1, 1, NULL, 0);
> > +
> > +		tmn.task = p;
> > +		tmn.from_cpu = old_cpu;
> > +		tmn.to_cpu = new_cpu;
> > +
> > +		atomic_notifier_call_chain(&task_migration_notifier, 0, &tmn);
> 
> We already have one event notifier there - look at the 
> perf_swcounter_event() callback. Why add a second one for essentially 
> the same thing?
> 
> We should only put a single callback there - a tracepoint defined via 
> TRACE_EVENT() - and any secondary users can register a callback to the 
> tracepoint itself.
> 
> There's many similar places in the kernel - with notifier chains and 
> also with a need to get tracepoints there. The fastest (and most 
> consistent) solution is to add just a single event callback facility.

But that would basically mandate tracepoints to be always enabled, do we
want to go there?

I don't think the overhead of tracepoints is understood well enough,
Jason you poked at that, do you have anything solid on that?

Also, I can imagine the embedded people to not want that.

I really like perf and tracepoints to not become co-dependent until
tracepoint become mandatory for all configurations.
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