Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

From: Jan Kara
Date: Fri Sep 18 2020 - 09:26:40 EST

On Fri 18-09-20 15:09:14, Oleg Nesterov wrote:
> On 09/18, Peter Zijlstra wrote:
> > > But again, do we really want this?
> >
> > I like the two counters better, avoids atomics entirely, some archs
> > hare horridly expensive atomics (*cough* power *cough*).
> I meant... do we really want to introduce percpu_up_read_irqsafe() ?
> Perhaps we can live with the fix from Hou? At least until we find a
> "real" performance regression.

I can say that for users of percpu rwsem in filesystems the cost of atomic
inc/dec is unlikely to matter. The lock hold times there are long enough
that it would be just lost in the noise.

For other stuff using them like get_online_cpus() or get_online_mems() I'm
not so sure...


Jan Kara <jack@xxxxxxxx>