linux-kernel - Re: [tip:x86/mm] percpu: Introduce a read-mostly percpu API

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Date:	Thu, 21 Oct 2010 07:33:50 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>
Subject: Re: [tip:x86/mm] percpu: Introduce a read-mostly percpu API

Le jeudi 21 octobre 2010 à 09:38 +0800, Shaohua Li a écrit :
> Hi hpa,
> On Thu, Oct 21, 2010 at 07:06:59AM +0800, tip-bot for Shaohua Li wrote:
> > [ hpa: moved it *after* the page-aligned section, for obvious
> >   reasons. ]
> move it after page-aligned section can't guarantee to avoid cache false sharing.
> we need:
> . = ALIGN(CACHE_LINE_SIZE);
> *(.data..percpu..readmostly)
> . = ALIGN(CACHE_LINE_SIZE);
> To make sure before/after readmostly section doesn't share cache line with
> others. The headcache is some arch need L1_CACHE_SIZE and others need
> (1 << INTERNODE_CACHE_SHIFT). We need change all ARCHs to make this happen.

Really, readmostly percpu variables dont need INTERNODE_CACHE_SHIFT, as
this is read mostly, and per cpu vars ;)

Just use L1_CACHE_BYTES

                *(.data..percpu..first)                           \
                *(.data..percpu..page_aligned)                    \
+               . = ALIGN(L1_CACHE_BYTES                          \
+               *(.data..percpu..readmostly)                      \
+               . = ALIGN(L1_CACHE_BYTES                          \
                *(.data..percpu)                                  \


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