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

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Message-ID: <20120521152345.GC7068@gmail.com>
Date:	Mon, 21 May 2012 17:23:48 +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:

> 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:

> 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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