Subject: Re: Regression seen for patch "sched:dont decrease idle sleep avg"
From: Mike Galbraith <efault@gmx.de>
To: Peter Williams <pwil3058@bigpond.net.au>
Cc: tim.c.chen@linux.intel.com, Con Kolivas <kernel@kolivas.org>,
       "Chen, Kenneth W" <kenneth.w.chen@intel.com>,
       linux-kernel@vger.kernel.org, mingo@elte.hu,
       Andrew Morton <akpm@osdl.org>
In-Reply-To: <446AAA6D.9080401@bigpond.net.au>
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	 <1147839913.8335.35.camel@homer>  <446AAA6D.9080401@bigpond.net.au>
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Date: Wed, 17 May 2006 07:24:41 +0200
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On Wed, 2006-05-17 at 14:45 +1000, Peter Williams wrote:
> Mike Galbraith wrote:
> > On Tue, 2006-05-16 at 16:32 -0700, Tim Chen wrote:
> >> On Tue, 2006-05-16 at 09:45 +1000, Con Kolivas wrote:
> >>
> >>> Yes it's only designed to detect something that has been asleep for an 
> >>> arbitrary long time and "categorised as idle"; it is not supposed to be a 
> >>> priority stepping stone for everything, in this case at MAX_BONUS-1. Mike 
> >>> proposed doing this instead, but it was never my intent. 
> >> It seems like just one sleep longer than INTERACTIVE_SLEEP is needed
> >> kick the priority of a process all the way to MAX_BONUS-1 and boost the
> >> sleep_avg, regardless of what the prior sleep_avg was.
> >>
> >> So if there is a cpu hog that has long sleeps occasionally, once it woke
> >> up, its priority will get boosted close to maximum, likely starving out
> >> other processes for a while till its sleep_avg gets reduced.  This
> >> behavior seems like something to avoid according to the original code
> >> comment.  Are we boosting the priority too quickly?  
> > 
> > The answer to that is, sometimes yes, and when it bites, it bites hard.
> > Happily, most hogs don't sleep much, and we don't generally have lots of
> > bursty sleepers.
> > 
> 
> But it's easy for a malicious user to exploit.  Yes?

Without limits, sure.  Burn malicious idiot's library card :)  The real
pain begins when you start examining legitimate loads.  It _can_ get
really and truly fugly.  Generally doesn't.

	-Mike

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