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SubjectRe: [PATCH v2 0/6] KVM: arm64: VCPU preempted check support
On 2020-01-13 12:12, Will Deacon wrote:
> [+PeterZ]
>
> On Thu, Dec 26, 2019 at 09:58:27PM +0800, Zengruan Ye wrote:
>> This patch set aims to support the vcpu_is_preempted() functionality
>> under KVM/arm64, which allowing the guest to obtain the VCPU is
>> currently running or not. This will enhance lock performance on
>> overcommitted hosts (more runnable VCPUs than physical CPUs in the
>> system) as doing busy waits for preempted VCPUs will hurt system
>> performance far worse than early yielding.
>>
>> We have observed some performace improvements in uninx benchmark
>> tests.
>>
>> unix benchmark result:
>> host: kernel 5.5.0-rc1, HiSilicon Kunpeng920, 8 CPUs
>> guest: kernel 5.5.0-rc1, 16 VCPUs
>>
>> test-case | after-patch |
>> before-patch
>> ----------------------------------------+-------------------+------------------
>> Dhrystone 2 using register variables | 334600751.0 lps |
>> 335319028.3 lps
>> Double-Precision Whetstone | 32856.1 MWIPS |
>> 32849.6 MWIPS
>> Execl Throughput | 3662.1 lps |
>> 2718.0 lps
>> File Copy 1024 bufsize 2000 maxblocks | 432906.4 KBps |
>> 158011.8 KBps
>> File Copy 256 bufsize 500 maxblocks | 116023.0 KBps |
>> 37664.0 KBps
>> File Copy 4096 bufsize 8000 maxblocks | 1432769.8 KBps |
>> 441108.8 KBps
>> Pipe Throughput | 6405029.6 lps |
>> 6021457.6 lps
>> Pipe-based Context Switching | 185872.7 lps |
>> 184255.3 lps
>> Process Creation | 4025.7 lps |
>> 3706.6 lps
>> Shell Scripts (1 concurrent) | 6745.6 lpm |
>> 6436.1 lpm
>> Shell Scripts (8 concurrent) | 998.7 lpm |
>> 931.1 lpm
>> System Call Overhead | 3913363.1 lps |
>> 3883287.8 lps
>> ----------------------------------------+-------------------+------------------
>> System Benchmarks Index Score | 1835.1 |
>> 1327.6
>
> Interesting, thanks for the numbers.
>
> So it looks like there is a decent improvement to be had from targetted
> vCPU
> wakeup, but I really dislike the explicit PV interface and it's already
> been
> shown to interact badly with the WFE-based polling in
> smp_cond_load_*().
>
> Rather than expose a divergent interface, I would instead like to
> explore an
> improvement to smp_cond_load_*() and see how that performs before we
> commit
> to something more intrusive. Marc and I looked at this very briefly in
> the
> past, and the basic idea is to register all of the WFE sites with the
> hypervisor, indicating which register contains the address being spun
> on
> and which register contains the "bad" value. That way, you don't bother
> rescheduling a vCPU if the value at the address is still bad, because
> you
> know it will exit immediately.
>
> Of course, the devil is in the details because when I say "address",
> that's
> a guest virtual address, so you need to play some tricks in the
> hypervisor
> so that you have a separate mapping for the lockword (it's enough to
> keep
> track of the physical address).
>
> Our hacks are here but we basically ran out of time to work on them
> beyond
> an unoptimised and hacky prototype:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pvcy
>
> Marc -- how would you prefer to handle this?

Let me try and rebase this thing to a modern kernel (I doubt it applies
without
conflicts to mainline). We can then have discussion about its merit on
the list
once I post it. It'd be good to have a pointer to the benchamrks that
have been
used here.

Thanks,

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

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