linux-kernel - Re: [RESEND RFC PATCH v1] arm64: kvm: flush tlbs by range in unmap_stage2

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Message-ID: <f74277fd-5af2-c46f-169f-c15a321165cd@huawei.com>
Date:   Mon, 27 Jul 2020 22:51:41 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <james.morse@....com>, <julien.thierry.kdev@...il.com>,
        <suzuki.poulose@....com>, <catalin.marinas@....com>,
        <will@...nel.org>, <steven.price@....com>, <mark.rutland@....com>,
        <ascull@...gle.com>, <kvm@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-mm@...ck.org>, <arm@...nel.org>, <xiexiangyou@...wei.com>
Subject: Re: [RESEND RFC PATCH v1] arm64: kvm: flush tlbs by range in
 unmap_stage2_range function

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