On Thu, May 08, 2014 at 06:42:14PM -0700, Davidlohr Bueso wrote: > On Thu, 2014-05-08 at 13:56 -0400, Jerome Glisse wrote: > > On Thu, May 08, 2014 at 07:47:04PM +0300, sagi grimberg wrote: > > > On 5/7/2014 5:33 AM, Davidlohr Bueso wrote: > > > >On Tue, 2014-05-06 at 12:29 +0200, Peter Zijlstra wrote: > > > >>So you forgot to CC Linus, Linus has expressed some dislike for > > > >>preemptible mmu_notifiers in the recent past: > > > >> > > > >> https://lkml.org/lkml/2013/9/30/385 > > > >I'm glad this came up again. > > > > > > > >So I've been running benchmarks (mostly aim7, which nicely exercises our > > > >locks) comparing my recent v4 for rwsem optimistic spinning against > > > >previous implementation ideas for the anon-vma lock, mostly: > > > > > > > >- rwsem (currently) > > > >- rwlock_t > > > >- qrwlock_t > > > >- rwsem+optspin > > > > > > > >Of course, *any* change provides significant improvement in throughput > > > >for several workloads, by avoiding to block -- there are more > > > >performance numbers in the different patches. This is fairly obvious. > > > > > > > >What is perhaps not so obvious is that rwsem+optimistic spinning beats > > > >all others, including the improved qrwlock from Waiman and Peter. This > > > >is mostly because of the idea of cancelable MCS, which was mimic'ed from > > > >mutexes. The delta in most cases is around +10-15%, which is non > > > >trivial. > > > > > > These are great news David! > > > > > > >I mention this because from a performance PoV, we'll stop caring so much > > > >about the type of lock we require in the notifier related code. So while > > > >this is not conclusive, I'm not as opposed to keeping the locks blocking > > > >as I once was. Now this might still imply things like poor design > > > >choices, but that's neither here nor there. > > > > > > So is the rwsem+opt strategy the way to go Given it keeps everyone happy? > > > We will be more than satisfied with it as it will allow us to > > > guarantee device > > > MMU update. > > > > > > >/me sees Sagi smiling ;) > > > > > > :) > > > > So i started doing thing with tlb flush but i must say things looks ugly. > > I need a new page flag (goodbye 32bits platform) and i need my own lru and > > page reclaimation for any page in use by a device, i need to hook up inside > > try_to_unmap or migrate (but i will do the former). I am trying to be smart > > by trying to schedule a worker on another cpu before before sending the ipi > > so that while the ipi is in progress hopefully another cpu might schedule > > the invalidation on the GPU and the wait after ipi for the gpu will be quick. > > > > So all in all this is looking ugly and it does not change the fact that i > > sleep (well need to be able to sleep). It just move the sleeping to another > > part. > > > > Maybe i should stress that with the mmu_notifier version it only sleep for > > process that are using the GPU those process are using userspace API like > > OpenCL which are not playing well with fork, ie read do not use fork if > > you are using such API. > > > > So for my case if a process has mm->hmm set to something that would mean > > that there is a GPU using that address space and that it is unlikely to > > go under the massive workload that people try to optimize the anon_vma > > lock for. > > > > My point is that with rwsem+optspin it could try spinning if mm->hmm > > was NULL and make the massive fork workload go fast, or it could sleep > > directly if mm->hmm is set. > > Sorry? Unless I'm misunderstanding you, we don't do such things. Our > locks are generic and need to work for any circumstance, no special > cases here and there... _specially_ with these kind of things. So no, > rwsem will spin as long as the owner is set, just like any other users. > > Thanks, > Davidlohr > I do not mind spining all time i was just thinking that it could be optimize away in case there is hmm for the current mm as it means that any way there very much likely gonna be a schedule inside the mmu_notifier. But if you prefer keep code generic i am fine with wasting cpu cycle. Cheers, Jérôme Glisse -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>