Messages in this thread | | | Date | Fri, 10 Feb 2006 16:25:55 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] percpu data: only iterate over possible CPUs |
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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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