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    SubjectRe: [RFC PATCH 0/3] generic hypercall support
    Paul E. McKenney wrote:
    > On Fri, May 08, 2009 at 08:43:40AM -0400, Gregory Haskins wrote:
    >
    >> Marcelo Tosatti wrote:
    >>
    >>> On Fri, May 08, 2009 at 10:59:00AM +0300, Avi Kivity wrote:
    >>>
    >>>
    >>>> Marcelo Tosatti wrote:
    >>>>
    >>>>
    >>>>> I think comparison is not entirely fair. You're using
    >>>>> KVM_HC_VAPIC_POLL_IRQ ("null" hypercall) and the compiler optimizes that
    >>>>> (on Intel) to only one register read:
    >>>>>
    >>>>> nr = kvm_register_read(vcpu, VCPU_REGS_RAX);
    >>>>>
    >>>>> Whereas in a real hypercall for (say) PIO you would need the address,
    >>>>> size, direction and data.
    >>>>>
    >>>>>
    >>>>>
    >>>> Well, that's probably one of the reasons pio is slower, as the cpu has
    >>>> to set these up, and the kernel has to read them.
    >>>>
    >>>>
    >>>>
    >>>>> Also for PIO/MMIO you're adding this unoptimized lookup to the
    >>>>> measurement:
    >>>>>
    >>>>> pio_dev = vcpu_find_pio_dev(vcpu, port, size, !in);
    >>>>> if (pio_dev) {
    >>>>> kernel_pio(pio_dev, vcpu, vcpu->arch.pio_data);
    >>>>> complete_pio(vcpu); return 1;
    >>>>> }
    >>>>>
    >>>>>
    >>>>>
    >>>> Since there are only one or two elements in the list, I don't see how it
    >>>> could be optimized.
    >>>>
    >>>>
    >>> speaker_ioport, pit_ioport, pic_ioport and plus nulldev ioport. nulldev
    >>> is probably the last in the io_bus list.
    >>>
    >>> Not sure if this one matters very much. Point is you should measure the
    >>> exit time only, not the pio path vs hypercall path in kvm.
    >>>
    >>>
    >> The problem is the exit time in of itself isnt all that interesting to
    >> me. What I am interested in measuring is how long it takes KVM to
    >> process the request and realize that I want to execute function "X".
    >> Ultimately that is what matters in terms of execution latency and is
    >> thus the more interesting data. I think the exit time is possibly an
    >> interesting 5th data point, but its more of a side-bar IMO. In any
    >> case, I suspect that both exits will be approximately the same at the
    >> VT/SVM level.
    >>
    >> OTOH: If there is a patch out there to improve KVMs code (say
    >> specifically the PIO handling logic), that is fair-game here and we
    >> should benchmark it. For instance, if you have ideas on ways to improve
    >> the find_pio_dev performance, etc.... One item may be to replace the
    >> kvm->lock on the bus scan with an RCU or something.... (though PIOs are
    >> very frequent and the constant re-entry to an an RCU read-side CS may
    >> effectively cause a perpetual grace-period and may be too prohibitive).
    >> CC'ing pmck.
    >>
    >
    > Hello, Greg!
    >
    > Not a problem. ;-)
    >
    > A grace period only needs to wait on RCU read-side critical sections that
    > started before the grace period started. As soon as those pre-existing
    > RCU read-side critical get done, the grace period can end, regardless
    > of how many RCU read-side critical sections might have started after
    > the grace period started.
    >
    > If you find a situation where huge numbers of RCU read-side critical
    > sections do indefinitely delay a grace period, then that is a bug in
    > RCU that I need to fix.
    >
    > Of course, if you have a single RCU read-side critical section that
    > runs for a very long time, that -will- delay a grace period. As long
    > as you don't do it too often, this is not a problem, though if running
    > a single RCU read-side critical section for more than a few milliseconds
    > is probably not a good thing. Not as bad as holding a heavily contended
    > spinlock for a few milliseconds, but still not a good thing.
    >

    Hey Paul,
    This makes sense, and it clears up a misconception I had about RCU.
    So thanks for that.

    Based on what Paul said, I think we can get some amount of gains in the
    PIO and PIOoHC stats from converting to RCU. I will do this next.

    -Greg



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