linux-kernel - Re: RCU NOHZ, tsc, and clock

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Message-ID: <20121014225135.GA3288@linux.vnet.ibm.com>
Date:	Sun, 14 Oct 2012 15:51:35 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	John Stultz <johnstul@...ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: RCU NOHZ, tsc, and clock_gettime

On Fri, Oct 12, 2012 at 02:27:01PM -0400, Prarit Bhargava wrote:
> 
> > The effect of removing the two functions you noted (on 3.6 and earlier)
> > is to prevent RCU from checking for dyntick-idle CPUs, likely incurring
> > a cache miss for each CPU with interrupts disabled.  If you have a lot
> > of CPUs (or even if NR_CPUS is large and you have a smaller number of
> > CPUs), this can result in user-space-visible delays.
> > 
> 
> Paul,
> 
> I built a kernel with NR_CPUS=48 and booted on a 48 cpu (logical) system.  I do
> not see a difference in the test -- the variance is AFAICT just as large as if I
> had run with NR_CPUS=4096.

OK -- have you applied John Stultz's suggestions?

							Thanx, Paul

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