Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu counter

Andrew Morton
2022-09-08T15:39:10Z

On Thu, 8 Sep 2022 16:25:47 +0800 "Sun, Jiebin" <jiebin.sun@intel.com> wrote:

> In our case, if the local
> percpu counter is near to INT_MAX and there comes a big msgsz, the
> overflow issue could happen.

percpu_counter_add_batch() handles this - your big message
won't overflow an s64.

Lookng at percpu_counter_add_batch(), is this tweak right?

- don't need to update *fbc->counters inside the lock
- that __this_cpu_sub() is an obscure way of zeroing the thing

--- a/lib/percpu_counter.c~a
+++ a/lib/percpu_counter.c
@@ -89,8 +89,8 @@ void percpu_counter_add_batch(struct per
unsigned long flags;
raw_spin_lock_irqsave(&fbc->lock, flags);
fbc->count += count;
- __this_cpu_sub(*fbc->counters, count - amount);
raw_spin_unlock_irqrestore(&fbc->lock, flags);
+ __this_cpu_write(*fbc->counters, 0);
} else {
this_cpu_add(*fbc->counters, amount);
}
_