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.

[root@ltcden8-lp8 ~]# ppc64_cpu —smt=4
[root@ltcden8-lp8 ~]# ppc64_cpu --info
Core   0:    0*    1*    2*    3*    4     5     6     7   
Core   1:    8*    9*   10*   11*   12    13    14    15   
Core   2:   16*   17*   18*   19*   20    21    22    23   
Core   3:   24*   25*   26*   27*   28    29    30    31   

Remove a core, SMT level is retained.

[root@ltcden8-lp8 ~]# ppc64_cpu --info
Core   0:    0*    1*    2*    3*    4     5     6     7  
Core   1:    8*    9*   10*   11*   12    13    14    15  
Core   2:   16*   17*   18*   19*   20    21    22    23  
[root@ltcden8-lp8 ~]#  

Add 3 cores, SMT level is not retained.

[  496.600648] Fallback order for Node 1: 1 0 
[  496.600655] Built 1 zonelists, mobility grouping on.  Total pages: 1228159
[  496.600676] Policy zone: Normal
[  496.661173] WARNING: workqueue cpumask: online intersect > possible intersect
[  499.530646] Fallback order for Node 3: 3 0 
[  499.530655] Built 2 zonelists, mobility grouping on.  Total pages: 1228159
[  499.530675] Policy zone: Normal

ppc64_cpu --info
Core   0:    0*    1*    2*    3*    4     5     6     7  
Core   1:    8*    9*   10*   11*   12    13    14    15  
Core   2:   16*   17*   18*   19*   20    21    22    23  
Core   3:   24*   25*   26*   27*   28*   29*   30*   31* 
Core   4:   32*   33*   34*   35*   36*   37*   38*   39* 
Core   5:   40*   41*   42*   43*   44*   45*   46*   47*
 [root@ltcden8-lp8 ~]# 

- Sachin