Sachin Sant [off-list ref] writes:

quoted

On 28-Jun-2023, at 3:35 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.

Thanks for the patches Laurent.

Is the SMT level retained even when dynamically changing SMT values?
I am observing difference in behaviour with and without smt-enabled
kernel command line option.

When smt-enabled= option is specified SMT level is retained across 
cpu core remove and add.

Without this option but changing SMT level during runtime using
ppc64_cpu —smt=<level>, the SMT level is not retained after
cpu core add.

That's because ppc64_cpu is not using the sysfs SMT control file, it's
just onlining/offlining threads manually.

If you run:
 $ ppc64_cpu --smt=4 

And then also do:

 $ echo 4 > /sys/devices/system/cpu/smt/control

It should work as expected?

ppc64_cpu will need to be updated to do that automatically.

cheers