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

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Date:	Wed, 14 Oct 2009 09:05:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Peter Zijlstra <peterz@...radead.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>
Subject: Re: [PATCH RFC] sched: add notifier for process migration

* 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.

	Ingo
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