On Fri, 15 Apr 2011, Tejun Heo wrote:
> > The preempt on/off seems to be a bigger deal for realtime.
>
> Also, the cmpxchg used is local one w/o LOCK prefix. It might not
> bring anything to table on !PREEMPT kernels but at the same time it
> shouldn't hurt either. One way or the other, some benchmark numbers
> showing that it at least doesn't hurt would be nice.
Maybe just fall back to this_cpu_write()?
> > > Maybe use here latest cmpxchg16b stuff instead and get rid of spinlock ?
> >
> > Shaohua already got an atomic in there. You mean get rid of his preempt
> > disable/enable in the slow path?
>
> I personally care much less about slow path. According to Shaohua,
> atomic64_t behaves pretty nice and it isn't too complex, so I'd like
> to stick with that unless complex this_cpu ops can deliver something
> much better.
Ok here is a new patch that will allow Shaohua to simply convert the
slowpath to a single atomic op. No preemption anymore anywhere.
Subject: percpu: preemptless __per_cpu_counter_add V3
Use this_cpu_cmpxchg to avoid preempt_disable/enable in __percpu_add.
V3 - separate out the slow path so that the slowpath can also be done with
a simple atomic add without preemption enable/disable.
- Fallback in the !PREEMPT case to a simple this_cpu_write().
Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>
---
lib/percpu_counter.c | 29 ++++++++++++++++++++---------
1 file changed, 20 insertions(+), 9 deletions(-)
Index: linux-2.6/lib/percpu_counter.c
===================================================================
--- linux-2.6.orig/lib/percpu_counter.c 2011-04-13 17:12:59.000000000 -0500
+++ linux-2.6/lib/percpu_counter.c 2011-04-15 12:34:39.000000000 -0500
@@ -71,19 +71,30 @@ EXPORT_SYMBOL(percpu_counter_set);
void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
{
- s64 count;
+ s64 count, new, overflow;
- preempt_disable();
- count = __this_cpu_read(*fbc->counters) + amount;
- if (count >= batch || count <= -batch) {
+ do {
+ overflow = 0;
+ count = this_cpu_read(*fbc->counters);
+
+ new = count + amount;
+ /* In case of overflow fold it into the global counter instead */
+ if (new >= batch || new <= -batch) {
+ overflow = new;
+ new = 0;
+ }
+#ifdef CONFIG_PREEMPT
+ } while (this_cpu_cmpxchg(*fbc->counters, count, new) != count);
+#else
+ } while (0);
+ this_cpu_write(*fbc->counters, new);
+#endif
+
+ if (unlikely(overflow)) {
spin_lock(&fbc->lock);
- fbc->count += count;
- __this_cpu_write(*fbc->counters, 0);
+ fbc->count += overflow;
spin_unlock(&fbc->lock);
- } else {
- __this_cpu_write(*fbc->counters, count);
}
- preempt_enable();
}
EXPORT_SYMBOL(__percpu_counter_add);
--
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