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

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Date:	Wed, 23 May 2012 12:16:29 +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 Tuesday, May 22, 2012 18:55:41 Vlad Zolotarov wrote:
> > > 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
> > ...
> 

Ingo, here is the proposal to the patch (series) description:

------------------------------------------------------------------------------------------------
Added "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. More specifically, on vSMP 
Foundation platform x86_cpu_to_apicid shared the same internode_cache_line 
with frequently written lapic_events.

>From the analysis of the first 33 per_cpu variables  out of 219 (memories they 
describe, to be more specific) the 8 have read_mostly nature 
(tlb_vector_offset, cpu_loops_per_jiffy, xen_debug_irq, etc.) and 25 are 
frequently written (irq_stack_union, gdt_page, exception_stacks, idt_desc, 
etc.). Assuming that the spread of the rest of the per_cpu variables is 
similar, identifying the read mostly memories will make more sense in terms of 
long-term code maintenance comparing to identifying frequently written 
memories.
---------------------------------------------------------------------------------------------------

Pls., tell me if the above looks satisfactory to u in light of all your 
previous remarks.

If yes - I'll respin the series with the description above.

thanks,
vlad
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