Re: [PATCH 16/23] sched: Use lightweight hazard pointers to grab lazy mms

[Linus Torvalds <torvalds@linux-foundation.org>]

[ Ugh, I actually went back and looked at Nick's patches again, to
just verify my memory, and they weren't as pretty as I thought they
were ]

On Sun, Jan 9, 2022 at 12:48 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I'd much rather have a *much* smaller patch that says "on x86 and
> powerpc, we don't need this overhead at all".

For some reason I thought Nick's patch worked at "last mmput" time and
the TLB flush IPIs that happen at that point anyway would then make
sure any lazy TLB is cleaned up.

But that's not actually what it does. It ties the
MMU_LAZY_TLB_REFCOUNT to an explicit TLB shootdown triggered by the
last mmdrop() instead. Because it really tied the whole logic to the
mm_count logic (and made lazy tlb to not do mm_count) rather than the
mm_users thing I mis-remembered it doing.

So at least some of my arguments were based on me just mis-remembering
what Nick's patch actually did (mainly because I mentally recreated
the patch from "Nick did something like this" and what I thought would
be the way to do it on x86).

So I guess I have to recant my arguments.

I still think my "get rid of lazy at last mmput" model should work,
and would be a perfect match for x86, but I can't really point to Nick
having done that.

So I was full of BS.

Hmm. I'd love to try to actually create a patch that does that "Nick
thing", but on last mmput() (ie when __mmput triggers). Because I
think this is interesting. But then I look at my schedule for the
upcoming week, and I go "I don't have a leg to stand on in this
discussion, and I'm just all hot air".

Because code talks, BS walks.

                Linus