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

[John Stultz]

On Thu, Jul 14, 2016 at 9:49 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 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?

So I am seeing synchronize_sched called, and its taking the
!rcu_gp_is_expedited path when I see the particularly bad latencies.

I wonder if I just mucked up applying the patch?

thanks
-john