linux-kernel - commit e9e9250b: sync wakeup bustage when waker is an RT task

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Message-Id: <1273924628.10630.24.camel@marge.simson.net>
Date:	Sat, 15 May 2010 13:57:08 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: commit e9e9250b: sync wakeup bustage when waker is an RT task

Hi Peter,

This commit excluded RT tasks from rq->load, was that intentional?  The
comment in struct rq states that load reflects *all* tasks, but since
this commit, that's no longer true.

Looking at lmbench lat_udp in a PREEMPT_RT kernel, I noticed that
wake_affine() is failing for sync wakeups when it should not.  It's
doing so because the waker in this case is an RT kernel thread
(sirq-net-rx) - we subtract the sync waker's weight, when it was never
added in the first place, resulting in this_load going gaga.  End result
is quite high latency numbers due to tasks jabbering cross-cache.

If the exclusion was intentional, I suppose I can do a waker class check
in wake_affine() to fix it.

	-Mike

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