Re: gpiolib gpio_chrdev_release duration is about 30 ms

[Kent Gibson <warthog618@xxxxxxxxx>]

On Fri, Jul 24, 2020 at 09:36:03PM +0200, Bartosz Golaszewski wrote:
> On Mon, Jul 20, 2020 at 10:14 AM Maxim Kochetkov <fido_max@xxxxxxxx> wrote:
> >
> > It works fine with this patch. Thank you so much!
> >
> > 18.07.2020 07:25, Kent Gibson пишет:
> > > On Fri, Jul 17, 2020 at 06:07:04PM +0300, Andy Shevchenko wrote:
> > >> On Fri, Jul 17, 2020 at 5:17 PM Maxim Kochetkov <fido_max@xxxxxxxx> wrote:
> > >>>
> > >>> I need a small userspace program to do some GPIO magic to communicate
> > >>> other hardware like devmem. This program takes about 2,5 seconds just to
> > >>> find GPIO lines by name.
> > >>>
> > >>> replacing synchronize_rcu to synchronize_rcu_expedited in
> > >>> atomic_notifier_chain_unregister gives the same boost as removing
> > >>> synchronize_rcu
> > >>
> > >> Have you tried to replace an atomic notifier call with a regular one?
> > >> IIRC it's still not clear why atomic is used there.
> > >>
> > >
> > > Indeed, I recently submitted a patch to switch the
> > > atomic_notifier_call_chain to blocking_notifier_call_chain, as some of
> > > the chained calls can sleep.
> > > Not sure if that is related, or if the change would make this case better
> > > or worse, but it would be interesting to find out.
> > > The patch is in the current gpio/devel, btw.
> > >
> > > Cheers,
> > > Kent.
> > >
> 
> Linus,
> 
> I think we should consider submitting this patch for stable then
> because this slow-down was affects previously existing use-cases.
> 

That patch is post the cdev split, so it certainly wont apply to an
older kernel.  I'm happy to backport it if you need it - just
nominate the branch you want it for.

Cheers,
Kent.