[lkml]   [2016]   [Jul]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes
On Wed, Jul 13, 2016 at 10:51:02PM +0200, Peter Zijlstra wrote:
> 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.

Do anyone other than the non-NO_HZ_FULL low-latency guys and the -rt
guys care?

> > 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.

Take the patch that I just sent out and make the choice of normal
vs. expedited depend on CONFIG_PREEMPT_RT or whatever the -rt guys are
calling it these days. Is there a low-latency Kconfig option other

The memory-barrier approach can definitely be made to work, but is
going to be more complex due to the need to wait for readers.

Thanx, Paul

 \ /
  Last update: 2016-07-13 23:41    [W:0.164 / U:8.264 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site