Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

[Andy Lutomirski]

On Fri, Jun 22, 2018 at 8:15 AM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> On Fri, 2018-06-22 at 08:04 -0700, Andy Lutomirski wrote:
> > On Wed, Jun 20, 2018 at 12:57 PM Rik van Riel <riel@xxxxxxxxxxx>
> > wrote:
> > >
> > > Lazy TLB mode can result in an idle CPU being woken up by a TLB
> > > flush,
> > > when all it really needs to do is reload %CR3 at the next context
> > > switch,
> > > assuming no page table pages got freed.
> > >
> > > This patch deals with that issue by introducing a third TLB state,
> > > TLBSTATE_FLUSH, which causes %CR3 to be reloaded at the next
> > > context
> > > switch.
> > >
> > > Atomic compare and exchange is used to close races between the TLB
> > > shootdown code and the context switch code. Keying off just the
> > > tlb_gen is likely to not be enough, since that would not give
> > > lazy_clb_can_skip_flush() information on when it is facing a race
> > > and has to send the IPI to a CPU in the middle of a LAZY -> OK
> > > switch.
> > >
> > > Unlike the 2016 version of this patch, CPUs in TLBSTATE_LAZY are
> > > not
> > > removed from the mm_cpumask(mm), since that would prevent the TLB
> > > flush IPIs at page table free time from being sent to all the CPUs
> > > that need them.
> >
> > Eek, this is so complicated.  In the 2016 version of the patches, you
> > needed all this.  But I rewrote the whole subsystem to make it easier
> > now :)  I think that you can get rid of all of this and instead just
> > revert the relevant parts of:
> >
> > b956575bed91ecfb136a8300742ecbbf451471ab
> >
> > All the bookkeeping is already in place -- no need for new state.
>
> I looked at using your .tlb_gen stuff, but we need a
> way to do that race free.  I suppose setting the
> tlbstate to !lazy before checking .tlb_gen might do
> the trick, if we get the ordering right at the tlb
> invalidation site, too?

Oh, right.

>
> Something like this:
>
> context switch                tlb invalidation
>
>                         advance mm->context.tlb_gen
>                         send IPI to cpus with !is_lazy tlb
>
>
> tlbstate.is_lazy = FALSE
> *need_flush = .tlb_gen < next_tlb_gen
>
> Do you see any holes in that?

Logically, is_lazy is (with your patches) just like mm_cpumask in
terms of ordering.  So I think your idea above is fine.  But I think
you need to make sure there's a full barrier between is_lazy = false
and reading .tlb_gen.

--Andy