On Dec 3, 2020, at 11:54 PM, Nicholas Piggin [off-list ref] wrote:

Excerpts from Andy Lutomirski's message of December 4, 2020 3:26 pm:

quoted

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