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

[David Woodhouse]

On 12 August 2026 17:20:51 BST, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>On Wed, Aug 12, 2026, David Woodhouse wrote:
>> 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.
>
>FWIW, the max latency and >8ms numbers are very appealing to me, as my concerns
>with using SRCU are all about the tail latencies.
>
>But I'm obviously not the one who'd be saddled with maintaining the code, so I'm
>more than a little biased towards choosing the more complex version.

Ack. Well, it isn't even *that* bad in terms of complexity. It's mostly just a while loop around the core of the try_synchronize_srcu() I posted before, and some syntactic sugar and safeguards.

Let's see what Paul thinks.



>> 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 │
>>   └───────────────┴─────────────────────┴───────────────────┴─────────────────────┘