Re: [PATCH 00/10] perf/uprobe: Optimize uprobes

[Matthew Wilcox]

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