linux-kernel - Re: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read

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Date:	Mon, 21 May 2012 22:19:58 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Vlad Zolotarov <vlad@...lemp.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>, Shai@...lemp.com,
	ido@...ery.com
Subject: Re: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read_mostly
 section


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

> On Monday, May 21, 2012 17:23:48 Ingo Molnar wrote:
> > * Vlad Zolotarov <vlad@...lemp.com> wrote:
> > > Pls., consider applying this patch series.
> > > 
> > > It contains the following changes:
> > >  - Adds two new macros DEFINE_EARLY_PER_CPU_READ_MOSTLY() and
> > >  
> > >    DECLARE_EARLY_PER_CPU_READ_MOSTLY().
> > >  
> > >  - Adds "read-mostly" qualifier to the following variables in smp.h:
> > >   - cpu_sibling_map
> > >   - cpu_core_map
> > >   - cpu_llc_shared_map
> > >   - cpu_llc_id
> > >   - cpu_number
> > >   - x86_cpu_to_apicid
> > >   - x86_bios_cpu_apicid
> > >   - x86_cpu_to_logical_apicid
> > > 
> > > As long as all the variables above are only written during the
> > > initialization, this change is meant to prevent the false
> > > sharing and improve the performance on large multiprocessor
> > > systems.
> > 
> > Why have you resent this? The feedback I gave has not been
> > 
> > addressed:
> 
> Hmmm... I'm a bit confused. There were two feedbacks/threads: 
> one on "Signed- off-by" format and the other where u asked for 
> a justification on a vSMP side.
> 
> The signed-off format sounded to me as a clear blocker for a 
> series so I fixed it and respined. I also mentioned it in 
> patch0.

Well, you need to address all blockers before we can proceed.

> The second thread seams like getting to submitting a separate 
> patch with a doc under Documents and vSMP testing results 
> explaining and justifying when and were per-CPU and/or 
> __read_mostly variables should be used.

No. As I said I'm not convinced that there are fewer read-mostly 
than read-write percpu variables. Please:

> 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.

I.e. *numbers* are needed: roughly how many percpu variables in 
a defconfig of one type versus the other type. This settles the 
question whether we want to identify read-mostly or 
write-frequently variables, to address this particular problem 
...

Thanks,

	Ingo
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