CONFIG_VIRT_CPU_ACCOUNTING_GEN under pseries does not provide stolen
time accounting unless CONFIG_PARAVIRT_TIME_ACCOUNTING is enabled.
Implement this using the VPA accumulated wait counters.

Note this will not work on current KVM hosts because KVM does not
implement the VPA dispatch counters (yet). It could be implemented
with the dispatch trace log as it is for VIRT_CPU_ACCOUNTING_NATIVE,
but that is not necessary for the more limited accounting provided
by PARAVIRT_TIME_ACCOUNTING, and it is more expensive, complex, and
has downsides like potential log wrap.

From Shrikanth:

  [...] it was tested on Power10 [PowerVM] Shared LPAR. system has two
  LPAR. we will call first one LPAR1 and second one as LPAR2. Test was
  carried out in SMT=1. Similar observation was seen in SMT=8 as well.

  LPAR config header from each LPAR is below. LPAR1 is twice as big as
  LPAR2. Since Both are sharing the same underlying hardware, work
  stealing will happen when both the LPAR's are contending for the same
  resource.

  LPAR1:
  type=Shared mode=Uncapped smt=Off lcpu=40 cpus=40 ent=20.00
  LPAR2:
  type=Shared mode=Uncapped smt=Off lcpu=20 cpus=40 ent=10.00

  mpstat was used to check for the utilization. stress-ng has been used
  as the workload. Few cases are tested. when the both LPAR are idle
  there is no steal time. when LPAR1 starts running at 100% which
  consumes all of the physical resource, steal time starts to get
  accounted.  With LPAR1 running at 100% and LPAR2 starts running, steal
  time starts increasing. This is as expected. When the LPAR2 Load is
  increased further, steal time increases further.

  Case 1: 0% LPAR1; 0% LPAR2
   %usr  %nice   %sys %iowait  %irq  %soft %steal %guest %gnice  %idle
   0.00   0.00   0.05   0.00   0.00   0.00   0.00   0.00   0.00  99.95

  Case 2: 100% LPAR1; 0% LPAR2
   %usr  %nice   %sys %iowait  %irq  %soft %steal %guest %gnice  %idle
  97.68   0.00   0.00   0.00   0.00   0.00   2.32   0.00   0.00   0.00

  Case 3: 100% LPAR1; 50% LPAR2
   %usr  %nice   %sys %iowait  %irq  %soft %steal %guest %gnice  %idle
  86.34   0.00   0.10   0.00   0.00   0.03  13.54   0.00   0.00   0.00

  Case 4: 100% LPAR1; 100% LPAR2
   %usr  %nice   %sys %iowait  %irq  %soft %steal %guest %gnice  %idle
  78.54   0.00   0.07   0.00   0.00   0.02  21.36   0.00   0.00   0.00

  Case 5: 50% LPAR1; 100% LPAR2
   %usr  %nice   %sys %iowait  %irq  %soft %steal %guest %gnice  %idle
  49.37   0.00   0.00   0.00   0.00   0.00   1.17   0.00   0.00  49.47

  Patch is accounting for the steal time and basic tests are holding
  good.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
[mpe: Add SPDX tag to new paravirt_api_clock.h]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220902085316.2071519-3-npiggin@gmail.com