Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation

On Wed, 2026-08-12 at 16:03 +0100, David Woodhouse wrote:
> On Wed, 2026-08-12 at 15:34 +0100, David Woodhouse wrote:
> > On Wed, 2026-08-12 at 15:05 +0100, David Woodhouse wrote:
> > > I'll rephrase that for my own understanding:
> > > 
> > > *If* we go all the way to building a whole SRCU flavour for this *and*
> > > implementing a spin-only variant of srcu_synchronize() which is
> > > tailored to the atomic-reader use case, *then* we don't need to remove
> > > the non_block_{start,end} guards around the MMU notifiers, which are
> > > basically never being called anyway and don't actually seem to protect
> > > against any real bugs.
> > > 
> > > Yes?
> > 
> > FWIW it looks something like this. I'll throw it into my torture and
> > latency tests, and we can see what Paul thinks of it. I'm still utterly
> > unconvinced it's needed, but I concede it has its good points.
> 
> This slightly refactored version is the one that's actually going into
> my torture tests...

Well, it survived first contact, and it's doing the soak testing now.

The average is basically no better than the try_synchronize_srcu()
case, unsurprisingly — as *both* of them just observe that there are no
readers and proceed immediately, in at least 99% of cases.

Like the existing rwlock case, it still manages double-digit p100
latency even when though *doesn't* actually sleep.

I don't *hate* it, but I do question the benefit of it over try-first.
Again, I'll defer to Paul, but personally I'd want to see a more
compelling use case for it.

  ┌───────────────┬─────────────────────┬───────────────────┬─────────────────────┐
  │               │      expedited      │     try-first     │       atomic        │
  ├───────────────┼─────────────────────┼───────────────────┼─────────────────────┤
  │ median drain  │ 32-128µs            │ 4-16µs            │ 4-16µs              │
  ├───────────────┼─────────────────────┼───────────────────┼─────────────────────┤
  │ avg           │ 118µs               │ 13.8µs            │ 12.3µs              │
  ├───────────────┼─────────────────────┼───────────────────┼─────────────────────┤
  │ >1ms          │ ~950ppm             │ ~990ppm           │ 838ppm              │
  ├───────────────┼─────────────────────┼───────────────────┼─────────────────────┤
  │ >8ms          │ 42ppm               │ 4.4ppm            │ 0.10ppm             │
  ├───────────────┼─────────────────────┼───────────────────┼─────────────────────┤
  │ max           │ 33.6ms              │ 17.6ms            │ 10.25ms             │
  ├───────────────┼─────────────────────┼───────────────────┼─────────────────────┤
  │ fallback rate │ —                   │ 1.2%              │ 0%                  │
  ├───────────────┼─────────────────────┼───────────────────┼─────────────────────┤
  │ sample        │ 32.6M drains, 10min │ 41M drains, 10min │ 40.5M drains, 10min │
  └───────────────┴─────────────────────┴───────────────────┴─────────────────────┘