Hi Vikram, On Fri, Jul 28, 2017 at 12:09:38PM -0700, Vikram Mulukutla wrote: > On 2017-07-28 02:28, Will Deacon wrote: > >On Thu, Jul 27, 2017 at 06:10:34PM -0700, Vikram Mulukutla wrote: > > <snip> > > >> > >>I think we should have this discussion now - I brought this up earlier > >>[1] > >>and I promised a test case that I completely forgot about - but here it > >>is (attached). Essentially a Big CPU in an acquire-check-release loop > >>will have an unfair advantage over a little CPU concurrently attempting > >>to acquire the same lock, in spite of the ticket implementation. If the > >>Big > >>CPU needs the little CPU to make forward progress : livelock. > >> > > <snip> > > >> > >>One solution was to use udelay(1) in such loops instead of cpu_relax(), > >>but > >>that's not very 'relaxing'. I'm not sure if there's something we could > >>do > >>within the ticket spin-lock implementation to deal with this. > > > >Does bodging cpu_relax to back-off to wfe after a while help? The event > >stream will wake it up if nothing else does. Nasty patch below, but I'd be > >interested to know whether or not it helps. > > > >Will > > > This does seem to help. Here's some data after 5 runs with and without the > patch.
Blimey, that does seem to make a difference. Shame it's so ugly! Would you be able to experiment with other values for CPU_RELAX_WFE_THRESHOLD? I had it set to 10000 in the diff I posted, but that might be higher than optimal. It would be interested to see if it correlates with num_possible_cpus() for the highly contended case. Will