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

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Date:	Sat, 15 May 2010 14:04:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: commit e9e9250b: sync wakeup bustage when waker is an RT task

On Sat, 2010-05-15 at 13:57 +0200, Mike Galbraith wrote:
> 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.

Right, because a static load value does not accurately reflect a RT task
which can run as long as it pretty well pleases. So instead we measure
the time spend running !fair tasks and scale down the cpu_power
proportionally.

> 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.

So basically make all RT wakeups sync?

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