lkml.org 
[lkml]   [2020]   [Jul]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RESEND RFC PATCH v1] arm64: kvm: flush tlbs by range in unmap_stage2_range function
Zhenyu,

On 2020-07-27 15:51, Zhenyu Ye wrote:
> 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>

This means nothing to me, I'm afraid.

>
> 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
> --------------+--------------+----------------------------------

I don't see how you turn a 1.184s reduction into a 6s gain.
Surely there is more to it than what you posted.

>>> +
>>> +    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.

I'm not sure this is applicable here, and it doesn't mean
this is as good on other systems.

Thanks,

M.
--
Jazz is not dead. It just smells funny...

\
 
 \ /
  Last update: 2020-07-27 19:13    [W:0.066 / U:0.580 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site