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

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