Messages in this thread | | | Subject | Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier | From | Rik van Riel <> | Date | Fri, 22 Jun 2018 11:15:51 -0400 |
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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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