Subject: [RFC v2 0/2] lazy mm refcounting

From: Andy Lutomirski <[email-masked]>
Date: Thu, 3 Dec 2020 21:26:15 -0800

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(-)

-- 
2.28.0