linux-kernel - Re: [PATCH] memcg: remove unneeded preempt

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Date:	Thu, 25 Aug 2011 09:34:06 -0700
From:	James Bottomley <James.bottomley@...senPartnership.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Thelen <gthelen@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] memcg: remove unneeded preempt_disable



Christoph Lameter <cl@...ux.com> wrote:

>On Thu, 25 Aug 2011, James Bottomley wrote:
>
>> On Thu, 2011-08-25 at 10:11 -0500, Christoph Lameter wrote:
>> > On Thu, 25 Aug 2011, Peter Zijlstra wrote:
>> >
>> > > On Thu, 2011-08-18 at 14:40 -0700, Andrew Morton wrote:
>> > > >
>> > > > I think I'll apply it, as the call frequency is low (correct?)
>and the
>> > > > problem will correct itself as other architectures implement
>their
>> > > > atomic this_cpu_foo() operations.
>> > >
>> > > Which leads me to wonder, can anything but x86 implement that
>this_cpu_*
>> > > muck? I doubt any of the risk chips can actually do all this.
>> > > Maybe Itanic, but then that seems to be dying fast.
>> >
>> > The cpu needs to have an RMW instruction that does something to a
>> > variable relative to a register that points to the per cpu base.
>> >
>> > Thats generally possible. The problem is how expensive the RMW is
>going to
>> > be.
>>
>> Risc systems generally don't have a single instruction for this,
>that's
>> correct.  Obviously we can do it as a non atomic sequence: read
>> variable, compute relative, read, modify, write ... but there's
>> absolutely no point hand crafting that in asm since the compiler can
>> usually work it out nicely.  And, of course, to have this atomic, we
>> have to use locks, which ends up being very expensive.
>
>ARM seems to have these LDREX/STREX instructions for that purpose which
>seem to be used for generating atomic instructions without lockes. I
>guess
>other RISC architectures have similar means of doing it?

Arm isn't really risc.  Most don't.  However even with ldrex/strex you need two instructions for rmw.

James
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