Subject: Re: [PATCH -tip-rcu] Make rcutorture more vicious: make quiescent
	rcutorture less power-hungry
From: Darren Hart <dvhltc@us.ibm.com>
To: paulmck@linux.vnet.ibm.com
Cc: Arjan van de Ven <arjan@infradead.org>, linux-kernel@vger.kernel.org,
       mingo@elte.hu, josh@freedesktop.org, niv@us.ibm.com, dino@in.ibm.com,
       akpm@linux-foundation.org, torvalds@linux-foundation.org,
       vegard.nossum@gmail.com, adobriyan@gmail.com, oleg@tv-sign.ru,
       bunk@kernel.org, rjw@sisk.pl
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Date: Mon, 23 Jun 2008 21:28:51 +0000
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On Mon, 2008-06-23 at 13:15 -0700, Paul E. McKenney wrote:
> On Mon, Jun 23, 2008 at 08:02:54PM +0000, Darren Hart wrote:
> > On Mon, 2008-06-23 at 11:07 -0700, Arjan van de Ven wrote:
> > > On Mon, 23 Jun 2008 17:54:09 +0000
> > > Darren Hart <dvhltc@us.ibm.com> wrote:
> > > 
> > > > I'm a little concerned about how this will affect real-time
> > > > performance, as queueing up lots of timers all at once can lead to
> > > > long running timer expiration handlers.  If just a schedule_timeout,
> > > > I suppose we are only looking at a process wakeup, as opposed to a
> > > > softirq context callback function?
> > > 
> > > in reality, the time it takes to deliver the interrupt (including
> > > waking the CPU up etc), is likely to be an order or two of magnitude
> > > higher than this kind of code loop.... 
> > 
> > Sure, if we just look at one of them.  Any idea how many such items
> > we're looking at rounding up to fire at the same time?  Is it dozens,
> > hundreds, thousands?
> 
> Hello, Darren,
> 
> Wouldn't these timers be running at low priority, so that high-priority
> realtime tasks would preempt them?

The timers run from softirq context will run at the priority of the
softirq, so it is configurable, and only tasks equal to or lower in
priority to that should see direct effects.

> 
> 							Thanx, Paul
-- 
Darren Hart
Real-Time Linux Team Lead
IBM Linux Technology Center

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