Re: [PATCH] percpu data: only iterate over possible CPUs

[Andrew Morton <akpm@osdl.org>]

Nathan Lynch <ntl@pobox.com> wrote:
>
> Eric Dumazet wrote:
> >
> > [PATCH] HOTPLUG_CPU : avoid hitting too many cachelines in recalc_bh_state()
> > 
> > Instead of using for_each_cpu(i), we can use for_each_online_cpu(i) : The 
> > difference matters if HOTPUG_CPU=y
> > 
> > When a CPU goes offline (ie removed from online map), it might have a non 
> > null bh_accounting.nr, so this patch adds a transfert of this counter to an 
> > online CPU counter.
> > 
> > We already have a hotcpu_notifier, (function buffer_cpu_notify()), where we 
> > can do this bh_accounting.nr transfert.
> > 
> > Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
> 
> > --- a/fs/buffer.c	2006-02-10 15:08:21.000000000 +0100
> > +++ b/fs/buffer.c	2006-02-10 15:47:55.000000000 +0100
> > @@ -3138,7 +3138,7 @@
> >  	if (__get_cpu_var(bh_accounting).ratelimit++ < 4096)
> >  		return;
> >  	__get_cpu_var(bh_accounting).ratelimit = 0;
> > -	for_each_cpu(i)
> > +	for_each_online_cpu(i)
> >  		tot += per_cpu(bh_accounting, i).nr;
> >  	buffer_heads_over_limit = (tot > max_buffer_heads);
> >  }
> > @@ -3187,6 +3187,9 @@
> >  		brelse(b->bhs[i]);
> >  		b->bhs[i] = NULL;
> >  	}
> > +	get_cpu_var(bh_accounting).nr += per_cpu(bh_accounting, cpu).nr ;
> > +	per_cpu(bh_accounting, cpu).nr = 0;
> > +	put_cpu_var(bh_accounting);
> >  }
> 
> But now there is a window between the time the cpu is marked offline
> and the time its bh_accounting.nr is moved to another cpu.  So
> recalc_bh_state could fail to set buffer_heads_over_limit when it
> should.

Yes, there is a wee race there, but the consequences of a glitch or small
inaccuracy in buffer_heads_over_limit will be transient and negligibe.

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