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From: Zhenyu Ye <yezhenyu2@huawei.com>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arch@vger.kernel.org, kvm@vger.kernel.org,
	catalin.marinas@arm.com, linux-kernel@vger.kernel.org,
	steven.price@arm.com, linux-mm@kvack.org, arm@kernel.org,
	will@kernel.org, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RESEND RFC PATCH v1] arm64: kvm: flush tlbs by range in unmap_stage2_range function
Date: Mon, 27 Jul 2020 22:51:41 +0800
Message-ID: <f74277fd-5af2-c46f-169f-c15a321165cd@huawei.com> (raw)
In-Reply-To: <5d54c860b3b4e7a98e4d53397e6424ae@kernel.org>

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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Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-24 13:43 Zhenyu Ye
2020-07-25 17:40 ` Marc Zyngier
2020-07-27 14:51   ` Zhenyu Ye [this message]
2020-07-27 17:12     ` Marc Zyngier

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