Re: [PATCH 2/2] powerpc: Enable ASYM_SMT on interleaved big-core systems

On Mon, May 14, 2018 at 01:22:07PM +1000, Michael Neuling wrote:

On Fri, 2018-05-11 at 16:47 +0530, Gautham R. Shenoy wrote:

quoted

From: "Gautham R. Shenoy" <redacted>

Each of the SMT4 cores forming a fused-core are more or less
independent units. Thus when multiple tasks are scheduled to run on
the fused core, we get the best performance when the tasks are spread
across the pair of SMT4 cores.

Since the threads in the pair of SMT4 cores of an interleaved big-core
are numbered {0,2,4,6} and {1,3,5,7} respectively, enable ASYM_SMT on
such interleaved big-cores that will bias the load-balancing of tasks
on smaller numbered threads, which will automatically result in
spreading the tasks uniformly across the associated pair of SMT4
cores.

Signed-off-by: Gautham R. Shenoy <redacted>
---
 arch/powerpc/kernel/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 9ca7148..0153f01 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c

@@ -1082,7 +1082,7 @@ static int powerpc_smt_flags(void)
 {
 	int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
 
-	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
+	if (cpu_has_feature(CPU_FTR_ASYM_SMT) || has_interleaved_big_core) {

Shouldn't we just set CPU_FTR_ASYM_SMT and leave this code

unchanged?

Yes, that would have the same effect. I refrained from doing that
since I thought CPU_FTR_ASYM_SMT has the "lower numbered threads
expedite thread-folding" connotation from the POWER7 generation.

If it is ok to overload CPU_FTR_ASYM_SMT, we can do what you suggest
and have all the changes in setup-common.c

--
Thanks and Regards
gautham.