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. 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. 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. What is the expected behaviour on x86 when enabling SMT and certain cores are offline? [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
