Messages in this thread |  | | Subject | Re: [RESEND RFC PATCH v1] arm64: kvm: flush tlbs by range in unmap_stage2_range function | From | Zhenyu Ye <> | Date | Mon, 27 Jul 2020 22:51:41 +0800 |
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Hi Marc,
On 2020/7/26 1:40, Marc Zyngier wrote: > On 2020-07-24 14:43, Zhenyu Ye wrote: >> Now in unmap_stage2_range(), we flush tlbs one by one just after the >> corresponding pages cleared. However, this may cause some performance >> problems when the unmap range is very large (such as when the vm >> migration rollback, this may cause vm downtime too loog). > > You keep resending this patch, but you don't give any numbers > that would back your assertion.
I have tested the downtime of vm migration rollback on arm64, and found the downtime could even take up to 7s. Then I traced the cost of unmap_stage2_range() and found it could take a maximum of 1.2s. The vm configuration is as follows (with high memory pressure, the dirty rate is about 500MB/s):
<memory unit='GiB'>192</memory> <vcpu placement='static'>48</vcpu> <memoryBacking> <hugepages> <page size='1' unit='GiB' nodeset='0'/> </hugepages> </memoryBacking>
After this patch applied, the cost of unmap_stage2_range() can reduce to 16ms, and VM downtime can be less than 1s.
The following figure shows a clear comparison:
| vm downtime | cost of unmap_stage2_range() --------------+--------------+---------------------------------- before change | 7s | 1200 ms after change | 1s | 16 ms --------------+--------------+----------------------------------
>> + >> + if ((end - start) >= 512 << (PAGE_SHIFT - 12)) { >> + __tlbi(vmalls12e1is); > > And what is this magic value based on? You don't even mention in the > commit log that you are taking this shortcut. >
If the page num is bigger than 512, flush all tlbs of this vm to avoid soft lock-ups on large TLB flushing ranges. Just like what the flush_tlb_range() does.
Thanks, Zhenyu
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