[PATCH 2/2] powerpc/topology: Override cpu_smt_mask

From: Srikar Dronamraju
Date: Mon Aug 03 2020 - 23:38:24 EST

On Power9 a pair of cores can be presented by the firmware as a big-core
for backward compatibility reasons, with 4 threads per (small) core and 8
threads per big-core. cpu_smt_mask() should generally point to the cpu mask
of the (small)core.

In order to maintain userspace backward compatibility (with Power8 chips in
case of Power9) in enterprise Linux systems, the topology_sibling_cpumask
has to be set to big-core. Hence override the default cpu_smt_mask() to be
powerpc specific allowing for better scheduling behaviour on Power.

Cc: linuxppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>
Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Michael Neuling <mikey@xxxxxxxxxxx>
Cc: Gautham R Shenoy <ego@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Vaidyanathan Srinivasan <svaidy@xxxxxxxxxxxxx>
Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
arch/powerpc/include/asm/cputhreads.h | 1 -
arch/powerpc/include/asm/smp.h | 13 +++++++++++++
2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/cputhreads.h b/arch/powerpc/include/asm/cputhreads.h
index deb99fd6e060..98c8bd155bf9 100644
--- a/arch/powerpc/include/asm/cputhreads.h
+++ b/arch/powerpc/include/asm/cputhreads.h
@@ -23,7 +23,6 @@
extern int threads_per_core;
extern int threads_per_subcore;
extern int threads_shift;
-extern bool has_big_cores;
extern cpumask_t threads_core_mask;
#define threads_per_core 1
diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
index 9cd0765633c5..d4bc28accb28 100644
--- a/arch/powerpc/include/asm/smp.h
+++ b/arch/powerpc/include/asm/smp.h
@@ -131,6 +131,19 @@ static inline struct cpumask *cpu_smallcore_mask(int cpu)

extern int cpu_to_core_id(int cpu);

+extern bool has_big_cores;
+#define cpu_smt_mask cpu_smt_mask
+static inline const struct cpumask *cpu_smt_mask(int cpu)
+ if (has_big_cores)
+ return per_cpu(cpu_smallcore_map, cpu);
+ return per_cpu(cpu_sibling_map, cpu);
+#endif /* CONFIG_SCHED_SMT */
/* Since OpenPIC has only 4 IPIs, we use slightly different message numbers.
* Make sure this matches openpic_request_IPIs in open_pic.c, or what shows up