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SubjectRe: [PATCH 04/10] lib: percpu_counter_mod64
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On Sat, 2007-04-21 at 02:55 -0700, Andrew Morton wrote:
> On Fri, 20 Apr 2007 17:51:58 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
> > Add percpu_counter_mod64() to allow large modifications.
> >
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > ---
> > include/linux/percpu_counter.h | 9 +++++++++
> > lib/percpu_counter.c | 28 ++++++++++++++++++++++++++++
> > 2 files changed, 37 insertions(+)
> >
> > Index: linux-2.6/include/linux/percpu_counter.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/percpu_counter.h 2007-04-12 13:54:55.000000000 +0200
> > +++ linux-2.6/include/linux/percpu_counter.h 2007-04-12 14:00:21.000000000 +0200
> > @@ -36,6 +36,7 @@ static inline void percpu_counter_destro
> > }
> >
> > void percpu_counter_mod(struct percpu_counter *fbc, s32 amount);
> > +void percpu_counter_mod64(struct percpu_counter *fbc, s64 amount);
> > s64 percpu_counter_sum(struct percpu_counter *fbc);
> >
> > static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> > @@ -81,6 +82,14 @@ percpu_counter_mod(struct percpu_counter
> > preempt_enable();
> > }
> >
> > +static inline void
> > +percpu_counter_mod64(struct percpu_counter *fbc, s64 amount)
> > +{
> > + preempt_disable();
> > + fbc->count += amount;
> > + preempt_enable();
> > +}
> > +
> > static inline s64 percpu_counter_read(struct percpu_counter *fbc)
> > {
> > return fbc->count;
> > Index: linux-2.6/lib/percpu_counter.c
> > ===================================================================
> > --- linux-2.6.orig/lib/percpu_counter.c 2006-07-31 13:07:38.000000000 +0200
> > +++ linux-2.6/lib/percpu_counter.c 2007-04-12 14:17:12.000000000 +0200
> > @@ -25,6 +25,34 @@ void percpu_counter_mod(struct percpu_co
> > }
> > EXPORT_SYMBOL(percpu_counter_mod);
> >
> > +void percpu_counter_mod64(struct percpu_counter *fbc, s64 amount)
> > +{
> > + long count;
> > + s32 *pcount;
> > + int cpu;
> > +
> > + if (amount >= FBC_BATCH || amount <= -FBC_BATCH) {
> > + spin_lock(&fbc->lock);
> > + fbc->count += amount;
> > + spin_unlock(&fbc->lock);
> > + return;
> > + }
>
> This is wrong, a little.
>
> If the counter was at -FBC_BATCH/2 and the caller passed in FBC_BATCH, we
> could just set the cpu-local counter to FBC_BATCH/2 instead of going for
> the lock.
>
> Probably doesn't matter though.

Right, I could have taken along the current percpu offset.

> > + cpu = get_cpu();
> > + pcount = per_cpu_ptr(fbc->counters, cpu);
> > + count = *pcount + amount;
> > + if (count >= FBC_BATCH || count <= -FBC_BATCH) {
> > + spin_lock(&fbc->lock);
> > + fbc->count += count;
> > + *pcount = 0;
> > + spin_unlock(&fbc->lock);
> > + } else {
> > + *pcount = count;
> > + }
> > + put_cpu();
> > +}
> > +EXPORT_SYMBOL(percpu_counter_mod64);
>
> Bloaty. Surely we won't be needing this on 32-bit kernels? Even monster
> PAE has only 64,000,000 pages and won't be using deltas of more than 4
> gigapages?
>
> <Does even 64-bit need to handle 4 gigapages in a single hit? /me suspects
> another changelog bug>

Yeah, /me chastises himself for that...

This is because percpu_counter is s64 instead of the native long; I need
to halve the counter at some point (bdi_writeout_norm) and do that by
subtracting half the current value.

If percpu_counter_mod is limited to s32 this might not always work
(although in practice it might just fit).



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