linux-kernel - Re: [PATCH 1/2] pseries/smp: export the smt level in the SYS FS.

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Date:   Tue, 18 Apr 2023 22:55:25 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        christophe.leroy@...roup.eu, msuchanek@...e.de,
        nathanl@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/2] pseries/smp: export the smt level in the SYS FS.

* Laurent Dufour <ldufour@...ux.ibm.com> [2023-04-13 17:38:51]:

> On 13/04/2023 15:37:59, Michael Ellerman wrote:
> > Hi Laurent,
> > 
> > Laurent Dufour <ldufour@...ux.ibm.com> writes:
> >> There is no SMT level recorded in the kernel neither in user space.
> >> Indeed there is no real constraint about that and mixed SMT levels are
> >> allowed and system is working fine this way.
> >>
> >> However when new CPU are added, the kernel is onlining all the threads
> >> which is leading to mixed SMT levels and confuse end user a bit.
> >>
> >> To prevent this exports a SMT level from the kernel so user space
> >> application like the energy daemon, could read it to adjust their settings.
> >> There is no action unless recording the value when a SMT value is written
> >> into the new sysfs entry. User space applications like ppc64_cpu should
> >> update the sysfs when changing the SMT level to keep the system consistent.
> >>
> >> Suggested-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> >> Signed-off-by: Laurent Dufour <ldufour@...ux.ibm.com>
> >> ---
> >>  arch/powerpc/platforms/pseries/pseries.h |  3 ++
> >>  arch/powerpc/platforms/pseries/smp.c     | 39 ++++++++++++++++++++++++
> >>  2 files changed, 42 insertions(+)
> > 
> > There is a generic sysfs interface for smt in /sys/devices/system/cpu/smt
> > 
> > I think we should be enabling that on powerpc and then adapting it to
> > our needs, rather than adding a pseries specific file.
> 
> Thanks Michael, I was not aware of this sysfs interface.
> 
> > Currently the generic code is only aware of SMT on/off, so it would need
> > to be taught about SMT4 and 8 at least.
> 
> Do you think we should limit our support to SMT4 and SMT8 only?

smt2 is also a valid already supported configuration and we are evaluating 
smt6 mode based on some inputs from ISV teams. 

So I believe having a value for all modes would be good. 

> 
> > There are already hooks in the generic code to check the SMT level when
> > bringing CPUs up, see cpu_smt_allowed(), they may work for the pseries
> > hotplug case too, though maybe we need some additional logic.
> > 
> > Wiring up the basic support is pretty straight forward, something like
> > the diff below.
> 

-- 
Thanks and Regards
Srikar Dronamraju