> 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. > > 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
