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

[Peter Zijlstra]

On Thu, Jul 14, 2016 at 03:18:09PM +0200, Peter Zijlstra 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.

Doesn't explode if I run:

root@ivb-ep:/cgroup# mkdir ponies; for ((i=0; i<60; i++)) ; do while :; do echo $$ > tasks; echo $$ > ponies/tasks ; done & done

with cgroup using either READER or WRITER bias.

So passes light torture.