Re: [PATCH] percpu: preemptless __per_cpu_counter_add

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 14 avril 2011 A  08:55 +0900, Tejun Heo a A(C)crit :
> Hello, Christoph.
> 
> On Wed, Apr 13, 2011 at 05:23:04PM -0500, Christoph Lameter wrote:
> > 
> > Suggested fixup. Return from slowpath and update percpu variable under
> > spinlock.
> > 
> > Signed-off-by: Christoph Lameter <cl@linux.com>
> > 
> > ---
> >  lib/percpu_counter.c |    8 ++------
> >  1 file changed, 2 insertions(+), 6 deletions(-)
> > 
> > Index: linux-2.6/lib/percpu_counter.c
> > ===================================================================
> > --- linux-2.6.orig/lib/percpu_counter.c	2011-04-13 17:20:41.000000000 -0500
> > +++ linux-2.6/lib/percpu_counter.c	2011-04-13 17:21:33.000000000 -0500
> > @@ -82,13 +82,9 @@ void __percpu_counter_add(struct percpu_
> >  			spin_lock(&fbc->lock);
> >  			count = __this_cpu_read(*fbc->counters);
> >  			fbc->count += count + amount;
> > +			__this_cpu_write(*fbc->counters, 0);
> >  			spin_unlock(&fbc->lock);
> > -			/*
> > -			 * If cmpxchg fails then we need to subtract the amount that
> > -			 * we found in the percpu value.
> > -			 */
> > -			amount = -count;
> > -			new = 0;
> > +			return;
> 
> Yeah, looks pretty good to me now.  Just a couple more things.
> 
> * Please fold this one into the original patch.
> 
> * While you're restructuring the functions, can you add unlikely to
>   the slow path?
> 
> It now looks correct to me but just in case, Eric, do you mind
> reviewing and acking it?
> 
> Thanks.
> 

I am not sure its worth it, considering we hit this on machines where
preemption is off (CONFIG_PREEMPT_NONE=y) ?



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