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

From: Tejun Heo
Date: Thu Jul 14 2016 - 08:08:54 EST

On Thu, Jul 14, 2016 at 02:04:28PM +0200, Peter Zijlstra wrote:
> > I think it probably makes sense to make this the default on !RT at
> > least with a separate patch w/o stable cc'd. While most use cases
> > will be fine with the latency on write path, it also means that the
> > reader side is blocked for the duration which can hurt. rwsem implies
> > a lot more readers and thus more read lock operations than writes.
> > It's weird to trade off higher latency for lower cpu usage when it
> > would also slow down all readers.
> NAK, no expedited muck by default. There's more than just RT that
> doesn't like IPI sprays.

Can you elaborate? If that's the case, we have the wrong implemention
for percpu-rwsem where very long delays for writers induce the same
level of delays to all readers. If expedited by default isn't
workable, we should move away from rcu_sync for percpu_rwsem.