linux-kernel - Re: [PATCH v3 0/2] Move x86_cpu_to_apicid to the __read

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Date:	Mon, 21 May 2012 16:54:25 +0300
From:	Vlad Zolotarov <vlad@...lemp.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"Shai Fultheim (Shai@...leMP.com)" <Shai@...lemp.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ido Yariv <ido@...ery.com>
Subject: Re: [PATCH v3 0/2] Move x86_cpu_to_apicid to the __read_mostly section

On Monday, May 21, 2012 02:32:46 PM Ingo Molnar wrote:
> * Shai Fultheim (Shai@...leMP.com) <Shai@...leMP.com> wrote:
> > Ingo,
> > 
> > The reason for this, as you pointed out, is the 'cache line'
> > size (4096 bytes).  We see significant false sharing is we do
> > not move this next to each other.
> 
> Which write-often variable caused the many cache flushes/fills?
> cpu_to_apicid is read mostly.
> 
> I.e. it might make more sense to identify the frequenty
> *modified* percpu variables, and move them to a separate
> section. I *think* most percpu variables are read mostly, so it
> would be more maintainable in the long run to figure out the
> frequently modified ones, not the frequently not modified ones.

I tend to disagree about the general claim that most per-CPU variables are 
read-mostly: consider the per-CPU data structures used in lock-less algorithms 
like softnet_data used in a NAPI. I'm not sure what is a more common - read-
only or not-read-only per-cpu data, but surely there are both...

In this specific patch we deal with something that is initialized once in the 
init time and then used as if it's a constant thus representing a clear 
"__read_mostly" case. 

Having said all that I think that the proposed solution, the one using 
__read_mostly infrastructure, is just ok both in a long run. I also doubt that 
we are currently facing a need to define an additional "frequently modified" 
section.

Pls., comment.

thanks,
vlad

> 
> Thanks,
> 
> 	Ingo
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