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

[Peter Zijlstra]

On Thu, Jul 14, 2016 at 09:43:40AM -0700, John Stultz wrote:
> On Thu, Jul 14, 2016 at 6:18 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Wed, Jul 13, 2016 at 10:51:02PM +0200, Peter Zijlstra wrote:
> >> So, IIRC, the trade-off is a full memory barrier in read_lock and
> >> read_unlock() vs sync_sched() in write.
> >>
> >> Full memory barriers are expensive and while the combined cost might
> >> well exceed the cost of the sync_sched() it doesn't suffer the latency
> >> issues.
> >>
> >> Not sure if we can frob the two in a single codebase, but I can have a
> >> poke if Oleg or Paul doesn't beat me to it.
> >
> > OK, not too horrible if I say so myself :-)
> >
> > The below is a compile tested only first draft so far. I'll go give it
> > some runtime next.
> 
> Unfortunately it didn't apply cleanly to the 4.4 based tree I'm
> working with, so I had to manually apply the entirety of the
> percpu-rwsem.c changes myself. Hopefully I didn't screw it up.
> 
> So running with this, I'm still seeing some pretty large delays. 80ms
> peak, with lots of >20ms values as well.
> So it doesn't seem to have the positive effect that Paul's change provided.

Well that is weird, did you put a tracepoint/printk in
synchronize_sched() to ensure we don't end up calling that?