Messages in this thread | | | Subject | Re: [PATCH 04/10] lib: percpu_counter_mod64 | From | Peter Zijlstra <> | Date | Sat, 21 Apr 2007 13:02:26 +0200 |
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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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