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

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Date:	Thu, 25 Aug 2011 11:31:33 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	James Bottomley <James.Bottomley@...senPartnership.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

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?




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