linux-kernel - Re: [git] CFS-devel, latest code

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Date:	Tue, 25 Sep 2007 11:40:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [git] CFS-devel, latest code


* Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com> wrote:

> On Tue, Sep 25, 2007 at 11:13:31AM +0200, Ingo Molnar wrote:
> > ok, i'm too seeing some sort of latency weirdness with 
> > CONFIG_FAIR_GROUP_SCHED enabled, _if_ there's Xorg involved which runs 
> > under root uid on my box - and hence gets 50% of all CPU time.
> > 
> > Srivatsa, any ideas? It could either be an accounting buglet (less 
> > likely, seems like the group scheduling bits stick to the 50% splitup 
> > nicely), or a preemption buglet. One potential preemption buglet would 
> > be for the group scheduler to not properly preempt a running task when a 
> > task from another uid is woken?
> 
> Yep, I noticed that too.
> 
> check_preempt_wakeup()
> {
> 	...
> 
> 	if (is_same_group(curr, p)) {
> 	    ^^^^^^^^^^^^^
> 
> 		resched_task();
> 	}
> 
> }
> 
> Will try a fix to check for preemption at higher levels ..

i bet fixing this will increase precision of group scheduling as well. 
Those long latencies can be thought of as noise as well, and the 
fair-scheduling "engine" might not be capable to offset all sources of 
noise. So generally, while we allow a certain amount of lag in 
preemption decisions (wakeup-granularity, etc.), with which the fairness 
engine will cope just fine, we do not want to allow unlimited lag.

	Ingo
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