On Thu, Jun 13, 2024 at 09:34:10PM GMT, Michael Ellerman wrote: > "Nysal Jan K.A." <ny...@linux.ibm.com> writes: > > From: "Nysal Jan K.A" <ny...@linux.ibm.com> > > > > After the addition of HOTPLUG_SMT support for PowerPC [1] there was a > > regression reported [2] when enabling SMT. > > This implies it was a kernel regression. But it can't be a kernel > regression because previously there was no support at all for the sysfs > interface on powerpc. > > IIUIC the regression was in the ppc64_cpu userspace tool, which switched > to using the new kernel interface without taking into account the way it > behaves. > > Or are you saying the kernel behaviour changed on x86 after the powerpc > HOTPLUG_SMT was added? >
The regression is in ppc64_cpu. If we need the older behaviour we will need this or an equivalent change in the kernel though. Fixing it in userspace in an efficient way might be difficult. > > On a system with at least > > one offline core, when enabling SMT, the expectation is that no CPUs > > of offline cores are made online. > > > > On a POWER9 system with 4 cores in SMT4 mode: > > $ ppc64_cpu --info > > Core 0: 0* 1* 2* 3* > > Core 1: 4* 5* 6* 7* > > Core 2: 8* 9* 10* 11* > > Core 3: 12* 13* 14* 15* > > > > Turn only one core on: > > $ ppc64_cpu --cores-on=1 > > $ ppc64_cpu --info > > Core 0: 0* 1* 2* 3* > > Core 1: 4 5 6 7 > > Core 2: 8 9 10 11 > > Core 3: 12 13 14 15 > > > > Change the SMT level to 2: > > $ ppc64_cpu --smt=2 > > $ ppc64_cpu --info > > Core 0: 0* 1* 2 3 > > Core 1: 4 5 6 7 > > Core 2: 8 9 10 11 > > Core 3: 12 13 14 15 > > > > As expected we see only two CPUs of core 0 are online > > > > Change the SMT level to 4: > > $ ppc64_cpu --smt=4 > > $ ppc64_cpu --info > > Core 0: 0* 1* 2* 3* > > Core 1: 4* 5* 6* 7* > > Core 2: 8* 9* 10* 11* > > Core 3: 12* 13* 14* 15* > > > > The CPUs of offline cores are made online. If a core is offline then > > enabling SMT should not online CPUs of this core. > > That's the way the ppc64_cpu tool behaves, but it's not necessarily what > other arches want. > True, but from a user perspective it seems logical though. I think one can make a case for either behaviour. > > An arch specific > > function topology_is_core_online() is proposed to address this. > > Another approach is to check the topology_sibling_cpumask() for any > > online siblings. This avoids the need for an arch specific function > > but is less efficient and more importantly this introduces a change > > in existing behaviour on other architectures. > > It's only x86 and powerpc right? > > Having different behaviour on the only two arches that support the > interface does not seem like a good result. > Agree, I was originally thinking of sending out a patch changing this for both architectures, but was unsure if there might be users who now depend on this behaviour on x86. > > What is the expected behaviour on x86 when enabling SMT and certain cores > > are offline? > > AFAIK no one really touches SMT on x86 other than to turn it off for > security reasons. > > cheers > Thanks for your comments. It will be good to hear if changing this behaviour for both x86 and PowerPC might be an acceptable path forward. Regards --Nysal > > [1] > > https://lore.kernel.org/lkml/20230705145143.40545-1-lduf...@linux.ibm.com/ > > [2] > > https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ > > > > Nysal Jan K.A (2): > > cpu/SMT: Enable SMT only if a core is online > > powerpc/topology: Check if a core is online > > > > arch/powerpc/include/asm/topology.h | 13 +++++++++++++ > > kernel/cpu.c | 12 +++++++++++- > > 2 files changed, 24 insertions(+), 1 deletion(-) > > > > > > base-commit: c760b3725e52403dc1b28644fb09c47a83cacea6 > > -- > > 2.35.3
