Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem

[Davidlohr Bueso]

On Tue, 04 Jul 2017, Thomas Gleixner wrote:

> On Tue, 4 Jul 2017, Davidlohr Bueso wrote:
> > On Tue, 04 Jul 2017, Davidlohr Bueso wrote:
> >
> > > As a side effect you end up optimizing get/put_online_mems() at the cost
> > > of more overhead for the actual hotplug operation, which is rare and of less
> > > performance importance.
> >
> > So nm this, the reader side actually gets _more_ expensive with pcpu-rwsems
> > due to at least two full barriers for each get/put operation.
>
> Compared to a mutex_lock/unlock() pair on a global mutex ....

Ah, right, I was thrown off the:

   if (mem_hotplug.active_writer == current)
      return;

checks, which is only true within hotplug_begin/end. So normally we'd take
the lock, which was my first impression. Sorry for the noise.

Thanks,
Davidlohr