linux-kernel - Re: [PATCH v2 0/9] Introduce SMT level and add PowerPC support

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Date:   Wed, 28 Jun 2023 21:03:28 +0530
From:   Sachin Sant <sachinp@...ux.ibm.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arch@...r.kernel.org, dave.hansen@...ux.intel.com,
        open list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        npiggin@...il.com, tglx@...utronix.de
Subject: Re: [PATCH v2 0/9]  Introduce SMT level and add PowerPC support



> On 28-Jun-2023, at 3:35 PM, Laurent Dufour <ldufour@...ux.ibm.com> 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@...den8-lp8 ~]# ppc64_cpu —smt=4
[root@...den8-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@...den8-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@...den8-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@...den8-lp8 ~]# 

- Sachin