Re: [RESEND RFC PATCH v1] arm64: kvm: flush tlbs by range in unmap_stage2_range function

From: Zhenyu Ye
Date: Mon Jul 27 2020 - 10:51:57 EST

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>
<page size='1' unit='GiB' nodeset='0'/>

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

>> +
>> +ÂÂÂ if ((end - start) >= 512 << (PAGE_SHIFT - 12)) {
>> +ÂÂÂÂÂÂÂ __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.