Messages in this thread | | | Date | Wed, 13 Jul 2016 22:51:02 +0200 | From | Peter Zijlstra <> | Subject | Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes |
| |
On Wed, Jul 13, 2016 at 04:39:44PM -0400, Tejun Heo wrote:
> > There is a synchronize_sched() in there, so sorta. That thing is heavily > > geared towards readers, as is the only 'sane' choice for global locks. > > It used to use the expedited variant until 001dac627ff3 > ("locking/percpu-rwsem: Make use of the rcu_sync infrastructure"), so > it might have been okay before then.
Right, but expedited stuff sprays IPIs around the entire system. That's stuff other people complain about.
> The options that I can see are > > 1. Somehow make percpu_rwsem's write behavior more responsive in a way > which is acceptable all use cases. This would be great but > probably impossible. > > 2. Add a fast-writer option to percpu_rwsem so that users which care > about write latency can opt in for higher processing overhead for > lower latency.
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.
| |