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

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Message-ID: <20120611090031.GM31556@gmail.com>
Date:	Mon, 11 Jun 2012 11:00:31 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Vlad Zolotarov <vlad@...lemp.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>, Shai@...lemp.com,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read_mostly
 section


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

> On Wednesday 23 May 2012 12:16:29 Vlad Zolotarov wrote:
> > 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.
> 
> Ingo, sorry for bothering. Could u, pls., tell if the above 
> description is ok? We'd like to move on with this patch 
> series.

Yeah, that description and analysis looks good and sensible.

Mind resending the updated patches in a new thread?

Thanks,

	Ingo
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