linux-kernel - Re: Netperf UDP_STREAM regression due to not sending IPIs in ttwu

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Date:	Wed, 10 Oct 2012 15:05:55 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Mike Galbraith <efault@....de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Netperf UDP_STREAM regression due to not sending IPIs in
 ttwu_queue()

On Wed, 2012-10-10 at 13:29 +0100, Mel Gorman wrote:
> Do we really switch more though?
> 
> Look at the difference in interrupts vs context switch. IPIs are an interrupt
> so if TTWU_QUEUE wakes process B using an IPI, does that count as a context
> switch?

Nope. Nor would it for NO_TTWU_QUEUE. A process waking another is just
that, a wakeup.

A context switch is when we stop running a process and start running
anther. A wakeup can lead to us deciding the newly woken task is a
better task to run, however its not a given.

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