Excerpts from Andy Lutomirski's message of December 5, 2020 12:37 am: > > >> On Dec 3, 2020, at 11:54 PM, Nicholas Piggin <npiggin@xxxxxxxxx> 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. I'd go the other way. The mm_cpumark, TLB, and lazy maintainence is different between architectures anyway. I'd keep the simple refcount, and the pretty simple shoot-lazies approaches for now at least until a bit more is done on other fronts. If x86 is shooting down lazies on the final TLB flush as well, then I might be inclined to think that's the better way to go in the long term. Shoot-lazies would be a bit of a bolted on hack for powerpc/hash but it has ~zero impact to core code really. Thanks, Nick
