Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes

From: Tejun Heo
Date: Wed Jul 13 2016 - 18:01:42 EST

Hello, Paul.

On Wed, Jul 13, 2016 at 02:18:41PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 13, 2016 at 05:05:26PM -0400, Tejun Heo wrote:
> > On Wed, Jul 13, 2016 at 02:03:15PM -0700, Paul E. McKenney wrote:
> > > Take the patch that I just sent out and make the choice of normal
> > > vs. expedited depend on CONFIG_PREEMPT_RT or whatever the -rt guys are
> > > calling it these days. Is there a low-latency Kconfig option other
> > > than CONFIG_NO_HZ_FULL?
> >
> > Sounds like a plan to me.
> I like the way we like each other's idea. Mutually assured laziness? ;-)

Heh, indeed. :)

Technically, I think the lglock approach would be better here given
the combination of requirements; however, it's quite a bit more code
which would likely require some sophistications down the line (like
blocking new readers first at the start of down_write). If we have to
go there, we'll go there but for now I think it'd be simpler to
conditionally switch to the expedited operations. It can be a config
option which is selected by !RT as you suggested. If anyone hits an
actual issue with that, we can go for the lglock thing.