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
