[RFC v2 0/2] lazy mm refcounting

From: Andy Lutomirski
Date: Fri Dec 04 2020 - 00:27:19 EST


This is part of a larger series here, but the beginning bit is irrelevant
to the current discussion:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/mm&id=203d39d11562575fd8bd6a094d97a3a332d8b265

This is IMO a lot better than v1. It's now almost entirely in generic
code. (It looks like it's 100% generic, but that's a lie -- the
generic code currently that all possible lazy mm refs are in
mm_cpumask(), and that's not true on all arches. So, if we take my
approach, we'll need to have a little arch hook to control this.)

Here's how I think it fits with various arches:

x86: On bare metal (i.e. paravirt flush unavailable), the loop won't do
much. The existing TLB shootdown when user tables are freed will
empty mm_cpumask of everything but the calling CPU. So x86 ends up
pretty close to as good as we can get short of reworking mm_cpumask() itself.

arm64: It needs the fixup above for correctness, but I think performance
should be pretty good. Compared to current kernels, we lose an mmgrab()
and mmdrop() on each lazy transition, and we add a reasonably fast loop
over all cpus on process exit. Someone (probably me) needs to make
sure we don't need some extra barriers.

power: Similar to x86.

s390x: Should be essentially the same as arm64.

Other arches: I don't know. Further research is required.

What do you all think?

Andy Lutomirski (2):
[NEEDS HELP] x86/mm: Handle unlazying membarrier core sync in the arch
code
[MOCKUP] sched/mm: Lightweight lazy mm refcounting

arch/x86/mm/tlb.c | 17 +++++-
kernel/fork.c | 4 ++
kernel/sched/core.c | 134 +++++++++++++++++++++++++++++++++++++------
kernel/sched/sched.h | 11 +++-
4 files changed, 145 insertions(+), 21 deletions(-)

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2.28.0