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

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Message-ID: <91dc3a14-4f1b-ebff-69d7-ff15469b5dcb@linux.ibm.com>
Date:   Thu, 29 Jun 2023 14:04:06 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Sachin Sant <sachinp@...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



Le 29/06/2023 à 13:10, Michael Ellerman a écrit :
> 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.

Hi Sachin and Michael,

Yes, ppc64_cpu will need an update, and I have a patch ready to be sent 
once this series will be accepted.

By the way, I've a fix for the build issue reported against the patch 
6/9. I'll send a v3 soon.

Cheers,

Laurent.