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

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Date:	Fri, 12 Oct 2012 14:27:01 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	paulmck@...ux.vnet.ibm.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


> 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.

P.
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