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

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Message-ID: <4AD5A9CC.4070907@redhat.com>
Date:	Wed, 14 Oct 2009 19:37:00 +0900
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, Jeremy Fitzhardinge <jeremy@...p.org>,
	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 10/14/2009 06:26 PM, Peter Zijlstra wrote:
>> 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.
>    

It would be cleanest to have both pvclock and tracepoints select 
migration notifiers, defaulting to off.  Similarly both perf and kvm 
should use preemption notifiers (they do the same thing - switch 
per-task values into and out of cpu registers).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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