Re: [Qemu-devel] About QEMU BQL and dirty log switch in Migration

[Wanpeng Li]

2017-05-11 22:18 GMT+08:00 Zhoujian (jay) <address@hidden>:
> Hi Wanpeng,
>
>> 2017-05-11 21:43 GMT+08:00 Wanpeng Li <address@hidden>:
>> > 2017-05-11 20:24 GMT+08:00 Paolo Bonzini <address@hidden>:
>> >>
>> >>
>> >> On 11/05/2017 14:07, Zhoujian (jay) wrote:
>> >>> -        * Scan sptes if dirty logging has been stopped, dropping
>> those
>> >>> -        * which can be collapsed into a single large-page spte.
>> Later
>> >>> -        * page faults will create the large-page sptes.
>> >>> +        * Reset each vcpu's mmu, then page faults will create the
>> large-page
>> >>> +        * sptes later.
>> >>>          */
>> >>>         if ((change != KVM_MR_DELETE) &&
>> >>>                 (old->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
>> >>> -               !(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
>> >>> -               kvm_mmu_zap_collapsible_sptes(kvm, new);
>> >
>> > This is an unlikely branch(unless guest live migration fails and
>> > continue to run on the source machine) instead of hot path, do you
>> > have any performance number for your real workloads?
>>
>> I find the original discussion by google.
>> https://lists.nongnu.org/archive/html/qemu-devel/2017-04/msg04143.html
>> You will not go to this branch if the guest live migration successfully.
>
>  In our tests, this branch is taken when living migration is successful.
>  AFAIK, the kmod does not know whether living migration successful or not
>  when dealing with KVM_SET_USER_MEMORY_REGION ioctl. Do I miss something?

Original there is a bug which will not clear memslot dirty log flag
after live migration fails, a patch is submitted to fix it,
https://lists.nongnu.org/archive/html/qemu-devel/2015-04/msg00794.html,
however, I can't remember whether the dirty log flag will be cleared
if live migration complete successfully at that time, but maybe not.
Paolo replied to the patch he has a better method. Then I'm too busy
and didn't follow the qemu patch for this fix any more, I just find
this commit is merged currently:
http://git.qemu.org/?p=qemu.git;a=commit;h=6f6a5ef3e429f92f987678ea8c396aab4dc6aa19.
This commit will clear memslot dirty log flag after live migration no
matter whether it is successful or not.

Regards,
Wanpeng Li