[RFC v2 2/2] [MOCKUP] sched/mm: Lightweight lazy mm refcounting

[Andy Lutomirski]

> On Dec 3, 2020, at 11:54 PM, Nicholas Piggin <npiggin at gmail.com> wrote:
> 
> Excerpts from Andy Lutomirski's message of December 4, 2020 3:26 pm:
>> This is a mockup.  It's designed to illustrate the algorithm and how the
>> code might be structured.  There are several things blatantly wrong with
>> it:
>> 
>> The coding stype is not up to kernel standards.  I have prototypes in the
>> wrong places and other hacks.
>> 
>> There's a problem with mm_cpumask() not being reliable.
> 
> Interesting, this might be a way to reduce those IPIs with fairly 
> minimal fast path cost. Would be interesting to see how much performance 
> advantage it has over my dumb simple shoot-lazies.

My real motivation isn’t really performance per se. I think there’s considerable value in keeping the core algorithms the same across all architectures, and I think my approach can manage that with only a single hint from the architecture as to which CPUs to scan.

With shoot-lazies, in contrast, enabling it everywhere would either malfunction or have very poor performance or even DoS issues on arches like arm64 and s390x that don’t track mm_cpumask at all.  I’m sure we could come up with some way to mitigate that, but I think that my approach may be better overall for keeping the core code uniform and relatively straightforward.

> 
> For powerpc I don't think we'd be inclined to go that way, so don't feel 
> the need to add this complexity for us alone -- we'd be more inclined to 
> move the exit lazy to the final TLB shootdown path, which we're slowly 
> getting more infrastructure in place to do.
> 


> 
> There's a few nits but I don't think I can see a fundamental problem 
> yet.

Thanks!

I can polish the patch, but I want to be sure the memory ordering parts are clear.

> 
> Thanks,
> Nick