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

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Date:   Thu, 29 Jun 2023 21:10:25 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Sachin Sant <sachinp@...ux.ibm.com>,
        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

Sachin Sant <sachinp@...ux.ibm.com> writes:
>> 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.

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