linux-kernel - Re: nohz: remove nohz_cpu

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Message-ID: <1310691105.28599.277.camel@debian>
Date:	Fri, 15 Jul 2011 08:51:45 +0800
From:	"Alex,Shi" <alex.shi@...el.com>
To:	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
Cc:	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Fu, Michael" <michael.fu@...el.com>
Subject: Re: nohz: remove nohz_cpu_mask

On Tue, 2011-07-12 at 01:30 +0800, Paul E. McKenney wrote:
> On Mon, Jul 11, 2011 at 08:47:05AM +0800, Alex,Shi wrote:
> > RCU didn't use this global variable now. Currently no user on it.
> 
> Indeed.  RCU's use of it turned out to be quite buggy.  :-(
> 
> > Since the ts->do_timer_last is not the real last periodic tick cpu in
> > most of time. I once want to compare the cpu_online_mask and
> > nohz_cpu_mask to get a real one, and than only let that cpu sleep
> > shorter, other cpu will try to sleep KTIME_MAX, that need a extra lock
> > for nohz_cpu_mask. But I checked my all platforms, from NHM-EX server to
> > laptops, all of them are waked up a few times per second. So, the
> > advantage is only in theory. 
> > 
> > Since no clear usage of this variable, why not remove it? That can save
> > a cache-line in all cpus and reduce atomic sync contention.
> 
> Works for me!
> 
> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 

Thomas, would you like to give some comments of this? 

Best regards!
Alex

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