[RFC PATCH 0/4] KVM: arm64: Improve efficiency of stage2 page table

[wangyanan (Y)]

On 2021/2/25 1:20, Alexandru Elisei wrote:
> Hi,
>
> On 2/24/21 2:35 AM, wangyanan (Y) wrote:
>
>> Hi Alex,
>>
>> On 2021/2/23 23:55, Alexandru Elisei wrote:
>>> Hi Yanan,
>>>
>>> I wanted to review the patches, but unfortunately I get an error when trying to
>>> apply the first patch in the series:
>>>
>>> Applying: KVM: arm64: Move the clean of dcache to the map handler
>>> error: patch failed: arch/arm64/kvm/hyp/pgtable.c:464
>>> error: arch/arm64/kvm/hyp/pgtable.c: patch does not apply
>>> error: patch failed: arch/arm64/kvm/mmu.c:882
>>> error: arch/arm64/kvm/mmu.c: patch does not apply
>>> Patch failed at 0001 KVM: arm64: Move the clean of dcache to the map handler
>>> hint: Use 'git am --show-current-patch=diff' to see the failed patch
>>> When you have resolved this problem, run "git am --continue".
>>> If you prefer to skip this patch, run "git am --skip" instead.
>>> To restore the original branch and stop patching, run "git am --abort".
>>>
>>> Tried this with Linux tags v5.11-rc1 to v5.11-rc7. It looks like pgtable.c and
>>> mmu.c from your patch is different than what is found on upstream master. Did you
>>> use another branch as the base for your patches?
>> Thanks for your attention.
>> Indeed, this series was  more or less based on the patches I post before (Link:
>> https://lore.kernel.org/r/20210114121350.123684-4-wangyanan55@huawei.com).
>> And they have already been merged into up-to-data upstream master (commit:
>> 509552e65ae8287178a5cdea2d734dcd2d6380ab), but not into tags v5.11-rc1 to
>> v5.11-rc7.
>> Could you please try the newest upstream master(since commit:
>> 509552e65ae8287178a5cdea2d734dcd2d6380ab) ? I have tested on my local and no
>> apply errors occur.
> That worked for me, thank you for the quick reply.
>
> Just to double check, when you run the benchmarks, the before results are for a
> kernel built from commit 509552e65ae8 ("KVM: arm64: Mark the page dirty only if
> the fault is handled successfully"), and the after results are with this series on
> top, right?

Yes, that's right. So the performance change results have nothing to do 
with the series of commit 509552e65ae8.

Thanks,

Yanan

>
> Thanks,
>
> Alex
>
>> Thanks,
>>
>> Yanan.
>>
>>> Thanks,
>>>
>>> Alex
>>>
>>> On 2/8/21 11:22 AM, Yanan Wang wrote:
>>>> Hi,
>>>>
>>>> This series makes some efficiency improvement of stage2 page table code,
>>>> and there are some test results to present the performance changes, which
>>>> were tested by a kvm selftest [1] that I have post:
>>>> [1] https://lore.kernel.org/lkml/20210208090841.333724-1-wangyanan55@huawei.com/
>>>>
>>>> About patch 1:
>>>> We currently uniformly clean dcache in user_mem_abort() before calling the
>>>> fault handlers, if we take a translation fault and the pfn is cacheable.
>>>> But if there are concurrent translation faults on the same page or block,
>>>> clean of dcache for the first time is necessary while the others are not.
>>>>
>>>> By moving clean of dcache to the map handler, we can easily identify the
>>>> conditions where CMOs are really needed and avoid the unnecessary ones.
>>>> As it's a time consuming process to perform CMOs especially when flushing
>>>> a block range, so this solution reduces much load of kvm and improve the
>>>> efficiency of creating mappings.
>>>>
>>>> Test results:
>>>> (1) when 20 vCPUs concurrently access 20G ram (all 1G hugepages):
>>>> KVM create block mappings time: 52.83s -> 3.70s
>>>> KVM recover block mappings time(after dirty-logging): 52.0s -> 2.87s
>>>>
>>>> (2) when 40 vCPUs concurrently access 20G ram (all 1G hugepages):
>>>> KVM creating block mappings time: 104.56s -> 3.70s
>>>> KVM recover block mappings time(after dirty-logging): 103.93s -> 2.96s
>>>>
>>>> About patch 2, 3:
>>>> When KVM needs to coalesce the normal page mappings into a block mapping,
>>>> we currently invalidate the old table entry first followed by invalidation
>>>> of TLB, then unmap the page mappings, and install the block entry at last.
>>>>
>>>> It will cost a lot of time to unmap the numerous page mappings, which means
>>>> the table entry will be left invalid for a long time before installation of
>>>> the block entry, and this will cause many spurious translation faults.
>>>>
>>>> So let's quickly install the block entry at first to ensure uninterrupted
>>>> memory access of the other vCPUs, and then unmap the page mappings after
>>>> installation. This will reduce most of the time when the table entry is
>>>> invalid, and avoid most of the unnecessary translation faults.
>>>>
>>>> Test results based on patch 1:
>>>> (1) when 20 vCPUs concurrently access 20G ram (all 1G hugepages):
>>>> KVM recover block mappings time(after dirty-logging): 2.87s -> 0.30s
>>>>
>>>> (2) when 40 vCPUs concurrently access 20G ram (all 1G hugepages):
>>>> KVM recover block mappings time(after dirty-logging): 2.96s -> 0.35s
>>>>
>>>> So combined with patch 1, it makes a big difference of KVM creating mappings
>>>> and recovering block mappings with not much code change.
>>>>
>>>> About patch 4:
>>>> A new method to distinguish cases of memcache allocations is introduced.
>>>> By comparing fault_granule and vma_pagesize, cases that require allocations
>>>> from memcache and cases that don't can be distinguished completely.
>>>>
>>>> ---
>>>>
>>>> Details of test results
>>>> platform: HiSilicon Kunpeng920 (FWB not supported)
>>>> host kernel: Linux mainline (v5.11-rc6)
>>>>
>>>> (1) performance change of patch 1
>>>> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 20
>>>>         (20 vcpus, 20G memory, block mappings(granule 1G))
>>>> Before patch: KVM_CREATE_MAPPINGS: 52.8338s 52.8327s 52.8336s 52.8255s 52.8303s
>>>> After  patch: KVM_CREATE_MAPPINGS:  3.7022s  3.7031s  3.7028s  3.7012s  3.7024s
>>>>
>>>> Before patch: KVM_ADJUST_MAPPINGS: 52.0466s 52.0473s 52.0550s 52.0518s 52.0467s
>>>> After  patch: KVM_ADJUST_MAPPINGS:  2.8787s  2.8781s  2.8785s  2.8742s  2.8759s
>>>>
>>>> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 40
>>>>         (40 vcpus, 20G memory, block mappings(granule 1G))
>>>> Before patch: KVM_CREATE_MAPPINGS: 104.560s 104.556s 104.554s 104.556s 104.550s
>>>> After  patch: KVM_CREATE_MAPPINGS:  3.7011s  3.7103s  3.7005s  3.7024s  3.7106s
>>>>
>>>> Before patch: KVM_ADJUST_MAPPINGS: 103.931s 103.936s 103.927s 103.942s 103.927s
>>>> After  patch: KVM_ADJUST_MAPPINGS:  2.9621s  2.9648s  2.9474s  2.9587s  2.9603s
>>>>
>>>> (2) performance change of patch 2, 3(based on patch 1)
>>>> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 1
>>>>         (1 vcpu, 20G memory, block mappings(granule 1G))
>>>> Before patch: KVM_ADJUST_MAPPINGS: 2.8241s 2.8234s 2.8245s 2.8230s 2.8652s
>>>> After  patch: KVM_ADJUST_MAPPINGS: 0.2444s 0.2442s 0.2423s 0.2441s 0.2429s
>>>>
>>>> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 20
>>>>         (20 vcpus, 20G memory, block mappings(granule 1G))
>>>> Before patch: KVM_ADJUST_MAPPINGS: 2.8787s 2.8781s 2.8785s 2.8742s 2.8759s
>>>> After  patch: KVM_ADJUST_MAPPINGS: 0.3008s 0.3004s 0.2974s 0.2917s 0.2900s
>>>>
>>>> cmdline: ./kvm_page_table_test -m 4 -t 2 -g 1G -s 20G -v 40
>>>>         (40 vcpus, 20G memory, block mappings(granule 1G))
>>>> Before patch: KVM_ADJUST_MAPPINGS: 2.9621s 2.9648s 2.9474s 2.9587s 2.9603s
>>>> After  patch: KVM_ADJUST_MAPPINGS: 0.3541s 0.3694s 0.3656s 0.3693s 0.3687s
>>>>
>>>> ---
>>>>
>>>> Yanan Wang (4):
>>>>     KVM: arm64: Move the clean of dcache to the map handler
>>>>     KVM: arm64: Add an independent API for coalescing tables
>>>>     KVM: arm64: Install the block entry before unmapping the page mappings
>>>>     KVM: arm64: Distinguish cases of memcache allocations completely
>>>>
>>>>    arch/arm64/include/asm/kvm_mmu.h | 16 -------
>>>>    arch/arm64/kvm/hyp/pgtable.c     | 82 +++++++++++++++++++++-----------
>>>>    arch/arm64/kvm/mmu.c             | 39 ++++++---------
>>>>    3 files changed, 69 insertions(+), 68 deletions(-)
>>>>
>>> .
> .