On Tue, Jul 09, 2024 at 04:29:43PM +0200, Peter Zijlstra wrote: > On Tue, Jul 09, 2024 at 07:11:23AM -0700, Paul E. McKenney wrote: > > On Tue, Jul 09, 2024 at 11:01:53AM +0200, Peter Zijlstra wrote: > > > On Mon, Jul 08, 2024 at 05:25:14PM -0700, Andrii Nakryiko wrote: > > > > > > > Quick profiling for the 8-threaded benchmark shows that we spend >20% > > > > in mmap_read_lock/mmap_read_unlock in find_active_uprobe. I think > > > > that's what would prevent uprobes from scaling linearly. If you have > > > > some good ideas on how to get rid of that, I think it would be > > > > extremely beneficial. > > > > > > That's find_vma() and friends. I started RCU-ifying that a *long* time > > > ago when I started the speculative page fault patches. I sorta lost > > > track of that effort, Willy where are we with that? Probably best to start with lock_vma_under_rcu() in mm/memory.c. > > > Specifically, how feasible would it be to get a simple RCU based > > > find_vma() version sorted these days? > > > > Liam's and Willy's Maple Tree work, combined with Suren's per-VMA locking > > combined with some of Vlastimil's slab work is pushing in that direction. > > I believe that things are getting pretty close. > > So I fundamentally do not believe in per-VMA locking. Specifically for > this case that would be trading one hot line for another. I tried > telling people that, but it doesn't seem to stick :/ SRCU also had its own performance problems, so we've got problems one way or the other. The per-VMA lock probably doesn't work quite the way you think it does, but it absoutely can be a hot cacheline. I did propose a store-free variant at LSFMM 2022 and again at 2023, but was voted down. https://lwn.net/Articles/932298/ I don't think the door is completely closed to a migration to that, but it's a harder sell than what we've got. Of course, data helps ... > Per VMA refcounts or per VMA locks are a complete fail IMO. > > I suppose I should go dig out the latest versions of those patches to > see where they're at :/ Merged in v6.4 ;-P
