linux-kernel - Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

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Date:	Tue, 10 Jun 2008 12:56:31 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 04/41] cpu ops: Core piece for generic atomic per cpu operations

On Tuesday 10 June 2008 09:54:09 Christoph Lameter wrote:
> On Tue, 10 Jun 2008, Rusty Russell wrote:
> > > > Yes, this should be fixed.  I thought i386 had optimized versions
> > > > pre-merge, but I was wrong (%gs for per-cpu came later, and noone
> > > > cleaned up these naive versions).  Did you want me to write them?
> > >
> > > How can that be fixed? You have no atomic instruction that calculates
> > > the per cpu address in one go.
> >
> > Huh?  "incl %fs:varname" does exactly this.
>
> Right that is what the cpu alloc patches do. So you could implement
> cpu_local_inc on top of some of the cpu alloc patches.

Or you could just implement it today as a standalone patch.

> > > And as long as that is the case you need to
> > > disable preempt. Otherwise you may increment the per cpu variable of
> > > another processor because the process was rescheduled after the address
> > > was calculated but before the increment was done.
> >
> > But of course, that is not a problem.  You make local_t an atomic_t, and
> > then it doesn't matter which CPU you incremented.
>
> But then the whole point of local_t is gone. Why not use atomic_t in the
> first place?

Because some archs can do better.

> > By definition if the caller cared, they would have had premption
> > disabled.
>
> There are numerous instances where the caller does not care about
> preemption. Its just important that one per cpu counter is increment in
> the least intrusive way. See f.e. the VM event counters.

Yes, and that's exactly the point.  The VM event counters are exactly a case 
where you should have used cpu_local_inc.

Rusty.
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