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

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Date:	Mon, 21 May 2012 16:08:22 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Vlad Zolotarov <vlad@...lemp.com>
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


* Vlad Zolotarov <vlad@...lemp.com> wrote:

> 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...

Well, a quick tally of percpu variables on a 'make defconfig' 
kernel would tell us one way or another?

Here there's almost 200 percpu variables active in the 64-bit 
x86 defconfig, and a quick random sample suggests that most are 
read-mostly.

I have no fundamental prefer to either approach, but the 
direction taken should be justified explicitly, with numbers, 
arguments, etc. - also a short blurb somewhere in the headers 
that explains when they should be used, so that others can be 
aware of vSMP's special needs here.

Thanks,

	Ingo
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