cpu_smt_mask tracks topology_sibling_cpumask. This would be good for
most architectures. One of the users of cpu_smt_mask(), would be to
identify idle-cores. On Power9, a pair of SMT4 cores can be presented by
the firmware as a SMT8 core for backward compatibility reasons.
Powerpc allows LPARs to be live migrated from Power8 to Power9. Do note
Power8 had only SMT8 cores. Existing software which has been
developed/configured for Power8 would expect to see SMT8 core.
Maintaining the illusion of SMT8 core is a requirement to make that
work.
In order to maintain above userspace backward compatibility with
previous versions of processor, Power9 onwards there is option to the
firmware to advertise a pair of SMT4 cores as a fused cores aka SMT8
core. On Power9 this pair shares the L2 cache as well. However, from the
scheduler's point of view, a core should be determined by SMT4, since
its a completely independent unit of compute. Hence allow PowerPc
architecture to override the default cpu_smt_mask() to point to the SMT4
cores in a SMT8 mode.
This will ensure the scheduler is always given the right information.
Cc: linuxppc-dev
Cc: LKML
Cc: Michael Ellerman
Cc: Michael Neuling
Cc: Gautham R Shenoy
Cc: Ingo Molnar
Cc: Peter Zijlstra
Cc: Valentin Schneider
Cc: Dietmar Eggemann
Cc: Mel Gorman
Cc: Vincent Guittot
Cc: Vaidyanathan Srinivasan
Acked-by; Peter Zijlstra (Intel)
Signed-off-by: Srikar Dronamraju
---
Changelog v1->v2:
Update the commit msg based on the discussion in community esp
with Peter Zijlstra and Michael Ellerman.
include/linux/topology.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/topology.h b/include/linux/topology.h
index 608fa4aadf0e..ad03df1cc266 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -198,7 +198,7 @@ static inline int cpu_to_mem(int cpu)
#define topology_die_cpumask(cpu) cpumask_of(cpu)
#endif
-#ifdef CONFIG_SCHED_SMT
+#if defined(CONFIG_SCHED_SMT) && !defined(cpu_smt_mask)
static inline const struct cpumask *cpu_smt_mask(int cpu)
{
return topology_sibling_cpumask(cpu);
--
2.18.2
