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

[Jay Zhou <jianjay.zhou@huawei.com>]

Hi Paolo and Wanpeng,

On 2017/5/17 16:38, Wanpeng Li wrote:
> 2017-05-17 15:43 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>>> Recently, I have tested the performance before migration and after migration failure
>>> using spec cpu2006 https://www.spec.org/cpu2006/, which is a standard performance
>>> evaluation tool.
>>>
>>> These are the steps:
>>> ======
>>>   (1) the version of kmod is 4.4.11(with slightly modified) and the version of
>>>   qemu is 2.6.0
>>>      (with slightly modified), the kmod is applied with the following patch
>>>
>>> diff --git a/source/x86/x86.c b/source/x86/x86.c
>>> index 054a7d3..75a4bb3 100644
>>> --- a/source/x86/x86.c
>>> +++ b/source/x86/x86.c
>>> @@ -8550,8 +8550,10 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>>>           */
>>>          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);
>>> +               !(new->flags & KVM_MEM_LOG_DIRTY_PAGES)) {
>>> +               printk(KERN_ERR "zj make KVM_REQ_MMU_RELOAD request\n");
>>> +               kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD);
>>> +       }
>>>
>>>          /*
>>>           * Set up write protection and/or dirty logging for the new slot.
>>
>> Try these modifications to the setup:
>>
>> 1) set up 1G hugetlbfs hugepages and use those for the guest's memory
>>
>> 2) test both without and with the above patch.
>>

In order to avoid random memory allocation issues, I reran the test cases:
(1) setup: start a 4U10G VM with memory preoccupied, each vcpu is pinned to a 
pcpu respectively, these resources(memory and pcpu) allocated to VM are all 
from NUMA node 0
(2) sequence: firstly, I run the 429.mcf of spec cpu2006 before migration, and 
get a result. And then, migration failure is constructed. At last, I run the 
test case again, and get an another result.
(3) results:
Host hugepages           THP on(2M)  THP on(2M)   THP on(2M)   THP on(2M)
Patch                    patch1      patch2       patch3       -
Before migration         No          No           No           Yes
After migration failed   Yes         Yes          Yes          No
Largepages               67->1862    62->1890     95->1865     1926
score of 429.mcf         189         188          188          189

Host hugepages           1G hugepages  1G hugepages  1G hugepages  1G hugepages
Patch                    patch1        patch2        patch3        -
Before migration         No            No            No            Yes
After migration failed   Yes           Yes           Yes           No
Largepages               21            21            26            39
score of 429.mcf         188           188           186           188

Notes:
patch1  means with "lazy collapse small sptes into large sptes" codes
patch2  means comment out "lazy collapse small sptes into large sptes" codes
patch3  means using kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD)
         instead of kvm_mmu_zap_collapsible_sptes(kvm, new)

"Largepages" means the value of /sys/kernel/debug/kvm/largepages

> In addition, we can compare /sys/kernel/debug/kvm/largepages w/ and
> w/o the patch. IIRC, /sys/kernel/debug/kvm/largepages will drop during
> live migration, it will keep a small value if live migration fails and
> w/o "lazy collapse small sptes into large sptes" codes, however, it
> will increase gradually if w/ the "lazy collapse small sptes into
> large sptes" codes.
>

No, without the "lazy collapse small sptes into large sptes" codes,
/sys/kernel/debug/kvm/largepages does drop during live migration,
but it still will increase gradually if live migration fails, see the result
above. I printed out the back trace when it increases after migration failure,

[139574.369098]  [<ffffffff81644a7f>] dump_stack+0x19/0x1b
[139574.369111]  [<ffffffffa02c3af6>] mmu_set_spte+0x2f6/0x310 [kvm]
[139574.369122]  [<ffffffffa02c4f7e>] __direct_map.isra.109+0x1de/0x250 [kvm]
[139574.369133]  [<ffffffffa02c8a76>] tdp_page_fault+0x246/0x280 [kvm]
[139574.369144]  [<ffffffffa02bf4e4>] kvm_mmu_page_fault+0x24/0x130 [kvm]
[139574.369148]  [<ffffffffa07c8116>] handle_ept_violation+0x96/0x170 [kvm_intel]
[139574.369153]  [<ffffffffa07cf949>] vmx_handle_exit+0x299/0xbf0 [kvm_intel]
[139574.369157]  [<ffffffff816559f0>] ? uv_bau_message_intr1+0x80/0x80
[139574.369161]  [<ffffffffa07cd5e0>] ? vmx_inject_irq+0xf0/0xf0 [kvm_intel]
[139574.369172]  [<ffffffffa02b35cd>] vcpu_enter_guest+0x76d/0x1160 [kvm]
[139574.369184]  [<ffffffffa02d9285>] ? kvm_apic_local_deliver+0x65/0x70 [kvm]
[139574.369196]  [<ffffffffa02bb125>] kvm_arch_vcpu_ioctl_run+0xd5/0x440 [kvm]
[139574.369205]  [<ffffffffa02a2b11>] kvm_vcpu_ioctl+0x2b1/0x640 [kvm]
[139574.369209]  [<ffffffff810e7852>] ? do_futex+0x122/0x5b0
[139574.369212]  [<ffffffff811fd9d5>] do_vfs_ioctl+0x2e5/0x4c0
[139574.369223]  [<ffffffffa02b0cf5>] ? kvm_on_user_return+0x75/0xb0 [kvm]
[139574.369225]  [<ffffffff811fdc51>] SyS_ioctl+0xa1/0xc0
[139574.369229]  [<ffffffff81654e09>] system_call_fastpath+0x16/0x1b

Any suggestion will be appreciated, Thanks!


Regards,
Jay Zhou