Il 06/08/2013 15:16, Xiao Guangrong ha scritto: > Hi Gleb, Paolo, Marcelo, Takuya, > > Any comments or further comments? :)
It's not the easiest patch to review. I've looked at it (beyond the small comments I have already posted), but it will take some time to digest it... By the way, both I and Gleb will be on vacation next week. I will read email, but I will not be able to apply patches or do pull requests. Paolo > 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(-) >> > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
