Re: [RFC PATCH 00/12] KVM: MMU: locklessly wirte-protect

[Xiao Guangrong]

Hi Gleb, Paolo, Marcelo, Takuya,

Any comments or further comments? :)

On 07/30/2013 09:01 PM, Xiao Guangrong wrote:
> Background
> ==========
> Currently, when mark memslot dirty logged or get dirty page, we need to
> write-protect large guest memory, it is the heavy work, especially, we need to
> hold mmu-lock which is also required by vcpu to fix its page table fault and
> mmu-notifier when host page is being changed. In the extreme cpu / memory used
> guest, it becomes a scalability issue.
> 
> This patchset introduces a way to locklessly write-protect guest memory.
> 
> Idea
> ==========
> There are the challenges we meet and the ideas to resolve them.
> 
> 1) How to locklessly walk rmap?
> The first idea we got to prevent "desc" being freed when we are walking the
> rmap is using RCU. But when vcpu runs on shadow page mode or nested mmu mode,
> it updates the rmap really frequently.
> 
> So we uses SLAB_DESTROY_BY_RCU to manage "desc" instead, it allows the object
> to be reused more quickly. We also store a "nulls" in the last "desc"
> (desc->more) which can help us to detect whether the "desc" is moved to anther
> rmap then we can re-walk the rmap if that happened. I learned this idea from
> nulls-list.
> 
> Another issue is, when a spte is deleted from the "desc", another spte in the
> last "desc" will be moved to this position to replace the deleted one. If the
> deleted one has been accessed and we do not access the replaced one, the
> replaced one is missed when we do lockless walk.
> To fix this case, we do not backward move the spte, instead, we forward move
> the entry: when a spte is deleted, we move the entry in the first desc to that
> position.
> 
> 2) How to locklessly access shadow page table?
> It is easy if the handler is in the vcpu context, in that case we can use
> walk_shadow_page_lockless_begin() and walk_shadow_page_lockless_end() that
> disable interrupt to stop shadow page be freed. But we are on the ioctl context
> and the paths we are optimizing for have heavy workload, disabling interrupt is
> not good for the system performance.
> 
> We add a indicator into kvm struct (kvm->arch.rcu_free_shadow_page), then use
> call_rcu() to free the shadow page if that indicator is set. Set/Clear the
> indicator are protected by slot-lock, so it need not be atomic and does not
> hurt the performance and the scalability.
> 
> 3) How to locklessly write-protect guest memory?
> Currently, there are two behaviors when we write-protect guest memory, one is
> clearing the Writable bit on spte and the another one is dropping spte when it
> points to large page. The former is easy we only need to atomicly clear a bit
> but the latter is hard since we need to remove the spte from rmap. so we unify
> these two behaviors that only make the spte readonly. Making large spte
> readonly instead of nonpresent is also good for reducing jitter.
> 
> And we need to pay more attention on the order of making spte writable, adding
> spte into rmap and setting the corresponding bit on dirty bitmap since
> kvm_vm_ioctl_get_dirty_log() write-protects the spte based on the dirty bitmap,
> we should ensure the writable spte can be found in rmap before the dirty bitmap
> is visible. Otherwise, we cleared the dirty bitmap and failed to write-protect
> the page.
> 
> Performance result
> ====================
> Host: CPU: Intel(R) Xeon(R) CPU           X5690  @ 3.47GHz x 12
> Mem: 36G
> 
> The benchmark i used and will be attached:
> a) kernbench
> b) migrate-perf
>    it emulates guest migration
> c) mmtest
>    it repeatedly writes the memory and measures the time and is used to
>    generate memory access in the guest which is being migrated
> d) Qemu monitor command to implement guest live migration
>    the script can be found in migrate-perf.
>   
> 
> 1) First, we use kernbench to benchmark the performance with non-write-protection
>   case to detect the possible regression:
> 
>   EPT enabled:  Base: 84.05      After the patch: 83.53
>   EPT disabled: Base: 142.57     After the patch: 141.70
> 
>   No regression and the optimization may come from lazily drop large spte.
> 
> 2) Benchmark the performance of get dirty page
>    (./migrate-perf -c 12 -m 3000 -t 20)
> 
>    Base: Run 20 times, Avg time:24813809 ns.
>    After the patch: Run 20 times, Avg time:8371577 ns.
>    
>    It improves +196%
>   
> 3) There is the result of Live Migration:
>    3.1) Less vcpus, less memory and less dirty page generated
>         (
>           Guest config: MEM_SIZE=7G        VCPU_NUM=6
>           The workload in migrated guest:
>           ssh -f $CLIENT "cd ~; rm -f result; nohup /home/eric/mmtest/mmtest -m 3000 -c 30 -t 60 > result &"
>         )
> 
>                Live Migration time (ms)   Benchmark (ns)
> ----------------------------------------+-------------+---------+
> EPT    | Baseline |     21638           |  266601028            |
>        + -------------------------------+-------------+---------+
>        |   After  |     21110    +2.5%  |  264966696    +0.6%   |
> ----------------------------------------+-------------+---------+
> Shadow | Baseline |     22542           |  271969284  |         |
>        +----------+---------------------+-------------+---------+
>        |  After   |     21641    +4.1%  |  270485511    +0.5%   |
> -------+----------+---------------------------------------------+
> 
>    3.2) More vcpus, more memory and less dirty page generated
>        (
>          Guest config: MEM_SIZE=25G VCPU_NUM=12
>          The workload in migrated guest:
>          ssh -f $CLIENT "cd ~; rm -f result; nohup /home/eric/mmtest/mmtest -m 15000 -c 30 -t 30 > result &"
>        )
> 
>                Live Migration time (ms)   Benchmark (ns)
> ----------------------------------------+-------------+---------+
> EPT    | Baseline |     72773           |  1278228350           |
>        + -------------------------------+-------------+---------+
>        |   After  |     70516     +3.2% |  1266581587   +0.9%   |
> ----------------------------------------+-------------+---------+
> Shadow | Baseline |     74198           |  1323180090 |         |
>        +----------+---------------------+-------------+---------+
>        |  After   |     64948   +14.2%  |  1299283302   +1.8%  |
> -------+----------+---------------------------------------------+
> 
>    3.3) Less vcpus, more memory and huge dirty page generated
>         ( 
>           Guest config: MEM_SIZE=25G VCPU_NUM=6
>           The workload in migrated guest:
>           ssh -f $CLIENT "cd ~; rm -f result; nohup /home/eric/mmtest/mmtest -m 15000 -c 30 -t 200 > result &"
>         )
> 
>                Live Migration time (ms)   Benchmark (ns)
> ----------------------------------------+-------------+---------+
> EPT    | Baseline |     267473          |  1224657502           |
>        + -------------------------------+-------------+---------+
>        |   After  |     267374   +0.03% |  1221520513   +0.6%   |
> ----------------------------------------+-------------+---------+
> Shadow | Baseline |     369999          |  1712004428 |         |
>        +----------+---------------------+-------------+---------+
>        |  After   |     335737   +10.2% |  1556065063   +10.2%  |
> -------+----------+---------------------------------------------+
> 
>    For the case of 3.3), EPT gets small benefit, the reason is only the first
>    time guest writes memory need take mmu-lock to mark spte from nonpresent to
>    present. Other writes cost lots of time to trigger the page fault due to
>    write-protection which are fixed by fast page fault which need not take
>    mmu-lock.
> 
> Xiao Guangrong (12):
>   KVM: MMU: remove unused parameter
>   KVM: MMU: properly check last spte in fast_page_fault()
>   KVM: MMU: lazily drop large spte
>   KVM: MMU: log dirty page after marking spte writable
>   KVM: MMU: add spte into rmap before logging dirty page
>   KVM: MMU: flush tlb if the spte can be locklessly modified
>   KVM: MMU: redesign the algorithm of pte_list
>   KVM: MMU: introduce nulls desc
>   KVM: MMU: introduce pte-list lockless walker
>   KVM: MMU: allow locklessly access shadow page table out of vcpu thread
>   KVM: MMU: locklessly write-protect the page
>   KVM: MMU: clean up spte_write_protect
> 
>  arch/x86/include/asm/kvm_host.h |  10 +-
>  arch/x86/kvm/mmu.c              | 442 ++++++++++++++++++++++++++++------------
>  arch/x86/kvm/mmu.h              |  28 +++
>  arch/x86/kvm/x86.c              |  19 +-
>  4 files changed, 356 insertions(+), 143 deletions(-)
> 

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