Re: [PATCH v2 0/6] KVM: arm64: VCPU preempted check support

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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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