Re: [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU schedule out

[Will Deacon <will@kernel.org>]

On Fri, Aug 06, 2021 at 12:24:36PM +0000, Shameerali Kolothum Thodi wrote:
> These are some test numbers with and without this patch, run on two
> different test setups.
> 
> 
> a)Test Setup -1
> -----------------------
> 
> Platform: HiSilicon D06 with 128 CPUs, VMID bits = 16
> Run 128 VMs concurrently each with 2 vCPUs. Each Guest will execute hackbench
> 5 times before exiting.
> 
> Measurements taken avg. of 10 Runs.
> 
> Image : 5.14-rc3
> ---------------------------
>   Time(s)       44.43813888
>   No. of exits    145,348,264
> 
> Image: 5.14-rc3 + vmid-v3
> ----------------------------------------
>   Time(s)        46.59789034
>   No. of exits     133,587,307
> 
> %diff against 5.14-rc3
>   Time: 4.8% more
>   Exits: 8% less 
> 
> Image: 5.14-rc3 + vmid-v3 + Without active_asid clear
> ---------------------------------------------------------------------------
>   Time(s)         44.5031782
>   No. of exits      144,443,188
> 
> %diff against 5.14-rc3
>   Time: 0.15% more
>   Exits: 2.42% less
> 
> b)Test Setup -2
> -----------------------
> 
> Platform: HiSilicon D06 + Kernel with maxcpus set to 8 and VMID bits set to 4.
> Run 40 VMs concurrently each with 2 vCPUs. Each Guest will execute hackbench
> 5 times before exiting.
> 
> Measurements taken avg. of 10 Runs.
> 
> Image : 5.14-rc3-vmid4bit
> ------------------------------------
>   Time(s)        46.19963266
>   No. of exits     23,699,546
> 
> Image: 5.14-rc3-vmid4bit + vmid-v3
> ---------------------------------------------------
>   Time(s)          45.83307736
>   No. of exits      23,260,203
> 
> %diff against 5.14-rc3-vmid4bit
>   Time: 0.8% less
>   Exits: 1.85% less 
> 
> Image: 5.14-rc3-vmid4bit + vmid-v3 + Without active_asid clear
> -----------------------------------------------------------------------------------------
>   Time(s)           44.5031782
>   No. of exits        144,443,188

Really? The *exact* same numbers as the "Image: 5.14-rc3 + vmid-v3 + Without
active_asid clear" configuration? Guessing a copy-paste error here.

> %diff against 5.14-rc3-vmid4bit
>   Time: 1.05% less
>   Exits: 2.06% less
> 
> As expected, the active_asid clear on schedule out is not helping.
> But without this patch, the numbers seems to be better than the
> vanilla kernel when we force the setup(cpus=8, vmd=4bits)
> to perform rollover.

I'm struggling a bit to understand these numbers. Are you saying that
clearing the active_asid helps in the 16-bit VMID case but not in the
4-bit case?

Why would the active_asid clear have any impact on the number of exits?

The problem I see with not having the active_asid clear is that we will
roll over more frequently as the number of reserved VMIDs increases.

Will