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

[Takuya Yoshikawa <takuya.yoshikawa@gmail.com>]

On Tue, 30 Jul 2013 21:01:58 +0800
Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> 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.

Nice improvements!

If I read the patch set correctly, this work contains the following changes:

Cleanups:
        Patch 1 and patch 12.

Lazy large page dropping for dirty logging:
        Patch 2-3.
        Patch 2 is preparatory to patch 3.

        This does not look like an RFC if you address Marcelo's comment.
        Any reason to include this in an RFC patch set?

Making remote TLBs flushable outside of mmu_lock for dirty logging:
        Patch 6.

        This is nice.  I'm locally using a similar patch for my work, but yours
        is much cleaner and better.  I hope this will get merged soon.

New Pte-list handling:
        Patch 7-9.

        Still reading the details.

RCU-based lockless write protection.
        Patch 10-11.

        If I understand RCU correctly, the current implementation has a problem:
        read-side critical sections can become too long.

        See the following LWN's article:
        "Sleepable RCU"
        https://lwn.net/Articles/202847/

        Especially, kvm_mmu_slot_remove_write_access() can take hundreds of
        milliseconds, or even a few seconds for guests using shadow paging.
        Is it possible to break the read-side critical section after protecting
        some pages? -- I guess so.

Anyway, I want to see the following non-RFC quality patches get merged first:
        - Lazy large page dropping for dirty logging:
        - Making remote TLBs flushable outside of mmu_lock for dirty logging

As you are doing in patch 11, the latter can eliminate the TLB flushes before
cond_resched_lock().  So this alone is an optimization, and since my work is
based on this TLB flush-less lock breaking, I would appriciate if you make this
change first in your clean way.

The remaining patches, pte-list refactoring and lock-less ones, also look
interesting, but I need to read more to understand them.

Thanks for the nice work!
        Takuya


> 
> 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(-)
> 
> -- 
> 1.8.1.4
> 
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-- 
Takuya Yoshikawa <takuya.yoshikawa@gmail.com>