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

[John Stultz]

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.

thanks
-john