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

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Date:	Mon, 21 May 2012 18:42:18 +0300
From:	Vlad Zolotarov <vlad@...lemp.com>
To:	Ingo Molnar <mingo@...nel.org>
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

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.

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. 

Pls., correct me if I misunderstood u and let me know what should I do next in 
order to make this patch series accepted.

thanks,
vlad


> > 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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