linux-kernel - Re: [RFC PATCHSET] sched,perf: unify tracers in sched and move perf on top of TP

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Date:	Thu, 6 May 2010 10:27:14 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, efault@....de,
	avi@...hat.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCHSET] sched,perf: unify tracers in sched and move perf
 on top of TP


* Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> On 05/06/2010 08:28 AM, Ingo Molnar wrote:
> > ( They are also under intense optimization - the jump-tracepoints patch makes 
> >   them probably even cheaper than preempt notifiers, in the off case. )
> 
> I mostly agree on other points but TPs and sched_notifiers are inherently 
> different in how they are enabled/disabled. sched_notifiers are 
> enabled/disabled per-task and at least w/ cmwq, there will always be some 
> tasks with active sched_notifiers so code level optimizations aren't really 
> useful.

Note that preempt notifiers used to modify scheduling behavior/semantics i'm 
really against. Please get that functionality into the scheduler - especially 
if it's essentially always-on.

Preempt notifiers are for things like KVM which want to do extended context 
saving. They are _NOT_ 'hooks' into the scheduler.

	Ingo
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