RE: [PATCH 8/8] shmem,percpu_counter: add _limited_add(fbc, limit, amount)

Hugh Dickins
2023-10-06T05:42:24Z

On Thu, 5 Oct 2023, Chen, Tim C wrote:

> >--- a/lib/percpu_counter.c
> >+++ b/lib/percpu_counter.c
> >@@ -278,6 +278,59 @@ int __percpu_counter_compare(struct
> >percpu_counter *fbc, s64 rhs, s32 batch) }
> >EXPORT_SYMBOL(__percpu_counter_compare);
> >
> >+/*
> >+ * Compare counter, and add amount if the total is within limit.
> >+ * Return true if amount was added, false if it would exceed limit.
> >+ */
> >+bool __percpu_counter_limited_add(struct percpu_counter *fbc,
> >+ s64 limit, s64 amount, s32 batch) {
> >+ s64 count;
> >+ s64 unknown;
> >+ unsigned long flags;
> >+ bool good;
> >+
> >+ if (amount > limit)
> >+ return false;
> >+
> >+ local_irq_save(flags);
> >+ unknown = batch * num_online_cpus();
> >+ count = __this_cpu_read(*fbc->counters);
> >+
> >+ /* Skip taking the lock when safe */
> >+ if (abs(count + amount) <= batch &&
> >+ fbc->count + unknown <= limit) {
> >+ this_cpu_add(*fbc->counters, amount);
> >+ local_irq_restore(flags);
> >+ return true;
> >+ }
> >+
> >+ raw_spin_lock(&fbc->lock);
> >+ count = fbc->count + amount;
> >+
>
> Perhaps we can fast path the case where for sure
> we will exceed limit?
>
> if (fbc->count + amount - unknown > limit)
> return false;

Thanks, that sounds reasonable: I'll try to add something like that -
but haven't thought about it carefully enough yet (too easy for me
to overlook some negative case which messes everything up).

Hugh