On 29-Jun-2023, at 8:01 PM, Laurent Dufour [off-list ref] wrote:

I'm taking over the series Michael sent previously [1] which is smartly
reviewing the initial series I sent [2].  This series is addressing the
comments sent by Thomas and me on the Michael's one.

Here is a short introduction to the issue this series is addressing:

When a new CPU is added, the kernel is activating all its threads. This
leads to weird, but functional, result when adding CPU on a SMT 4 system
for instance.

Here the newly added CPU 1 has 8 threads while the other one has 4 threads
active (system has been booted with the 'smt-enabled=4' kernel option):

ltcden3-lp12:~ # ppc64_cpu --info
Core   0:    0*    1*    2*    3*    4     5     6     7
Core   1:    8*    9*   10*   11*   12*   13*   14*   15*

This mixed SMT level may confused end users and/or some applications.

There is no SMT level recorded in the kernel (common code), neither in user
space, as far as I know. Such a level is helpful when adding new CPU or
when optimizing the energy efficiency (when reactivating CPUs).

When SMP and HOTPLUG_SMT are defined, this series is adding a new SMT level
(cpu_smt_num_threads) and few callbacks allowing the architecture code to
fine control this value, setting a max and a "at boot" level, and
controling whether a thread should be onlined or not.

v3:
 Fix a build error in the patch 6/9

Successfully tested the V3 version on a Power10 LPAR. Add/remove of
processor core worked correctly, preserving the SMT level (on a kernel
booted with smt-enabled= parameter)

Laurent (Thanks!) also provided a patch to update the ppc64_cpu &
lparstat utility. With patched ppc64_cpu utility verified that SMT level
changed at runtime was preserved across processor core add (on
a kernel booted without smt-enabled= parameter)

Based on these test results

Tested-by: Sachin Sant <redacted>

- Sachin