lkml.org 
[lkml]   [2012]   [May]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault
    On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote:

    > On Fri, 27 Apr 2012 11:52:13 -0300
    > Marcelo Tosatti <mtosatti@redhat.com> wrote:
    >
    >> Yes but the objective you are aiming for is to read and write sptes
    >> without mmu_lock. That is, i am not talking about this patch.
    >> Please read carefully the two examples i gave (separated by "example)").
    >
    > The real objective is not still clear.
    >
    > The ~10% improvement reported before was on macro benchmarks during live
    > migration. At least, that optimization was the initial objective.
    >
    > But at some point, the objective suddenly changed to "lock-less" without
    > understanding what introduced the original improvement.
    >
    > Was the problem really mmu_lock contention?
    >


    Takuya, i am so tired to argue the advantage of lockless write-protect
    and lockless O(1) dirty-log again and again.

    > If the path being introduced by this patch is really fast, isn't it
    > possible to achieve the same improvement still using mmu_lock?
    >
    >
    > Note: During live migration, the fact that the guest gets faulted is
    > itself a limitation. We could easily see noticeable slowdown of a
    > program even if it runs only between two GET_DIRTY_LOGs.
    >


    Obviously no.

    It depends on what the guest is doing, from my autotest test, it very
    easily to see that, the huge improvement is on bench-migration not
    pure-migration.

    >
    >> The rules for code under mmu_lock should be:
    >>
    >> 1) Spte updates under mmu lock must always be atomic and
    >> with locked instructions.
    >> 2) Spte values must be read once, and appropriate action
    >> must be taken when writing them back in case their value
    >> has changed (remote TLB flush might be required).
    >
    > Although I am not certain about what will be really needed in the
    > final form, if this kind of maybe-needed-overhead is going to be
    > added little by little, I worry about possible regression.


    Well, will you suggest Linus to reject all patches and stop
    all discussion for the "possible regression" reason?



    \
     
     \ /
      Last update: 2012-05-02 08:21    [W:0.026 / U:31.388 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site