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

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Message-Id: <1273943222.8752.7.camel@marge.simson.net>
Date:	Sat, 15 May 2010 19:07:02 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
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 14:04 +0200, Peter Zijlstra wrote:
> 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?

I was going to just skip subtracting waker's weight ala

        /*
         * If sync wakeup then subtract the (maximum possible)
         * effect of the currently running task from the load
         * of the current CPU:
         */
	if (sync && !task_has_rt_policy(curr))
		...

	-Mike

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