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

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Message-ID: <20101021061701.GA17054@sli10-conroe.sh.intel.com>
Date:	Thu, 21 Oct 2010 14:17:01 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Eric Dumazet <eric.dumazet@...il.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

On Thu, Oct 21, 2010 at 02:09:47PM +0800, H. Peter Anvin wrote:
> On 10/20/2010 11:07 PM, Eric Dumazet wrote:
> > Le mercredi 20 octobre 2010 à 22:54 -0700, H. Peter Anvin a écrit :
> > 
> >> L1_CACHE_BYTES is completely pointless, since if there is sharing to
> >> worry about *at all*, it's probably at the L2 or L3 cache levels.
> >>
> > 
> > I see, and we dont have better way to express this hint/requirement than
> > using PAGE_SIZE or INTERNODE_CACHE_SHIFT ?
> > 
> > In your patch you force a PAGE_SIZE alignement *before*
> > *(.data..percpu..page_aligned)   
> > 
> > If this alignment is really needed, this should be part of another
> > patch, since this fixes a previous bug in 2.6.36 ?
> > 
> 
> It was part of Shaohua's patch... I don't know if it does anything
> useful, but it also doesn't hurt.
This isn't a previous bug. see
#define DECLARE_PER_CPU_PAGE_ALIGNED(type, name)			\
	DECLARE_PER_CPU_SECTION(type, name, "..page_aligned")		\
	__aligned(PAGE_SIZE)
the ..page_aligned is already page aligned. I add it is because it can make
the .readmostly section guarantee to have no cache false sharing, because
I add the . = ALIGN(PAGE_SIZE); before .readmostly section, and .page_aligned
follows, but for sure this wates some memory.
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