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    SubjectRe: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier
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    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@surriel.com>
    > 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?

    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?

    I will gladly simplify this code and get rid
    of the atomic operations :)



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