Re: [PATCH 10/11] powerpc/smp: Implement cpu_to_coregroup_id

* Gautham R Shenoy [off-list ref] [2020-07-17 13:56:53]:

On Tue, Jul 14, 2020 at 10:06:23AM +0530, Srikar Dronamraju wrote:

quoted

Lookup the coregroup id from the associativity array.

If unable to detect the coregroup id, fallback on the core id.
This way, ensure sched_domain degenerates and an extra sched domain is
not created.

Ideally this function should have been implemented in
arch/powerpc/kernel/smp.c. However if its implemented in mm/numa.c, we
don't need to find the primary domain again.

If the device-tree mentions more than one coregroup, then kernel
implements only the last or the smallest coregroup, which currently
corresponds to the penultimate domain in the device-tree.

Cc: linuxppc-dev <redacted>
Cc: Michael Ellerman <redacted>
Cc: Nick Piggin <redacted>
Cc: Oliver OHalloran <redacted>
Cc: Nathan Lynch <redacted>
Cc: Michael Neuling <redacted>
Cc: Anton Blanchard <redacted>
Cc: Gautham R Shenoy <redacted>
Cc: Vaidyanathan Srinivasan <redacted>
Signed-off-by: Srikar Dronamraju <redacted>
---
 arch/powerpc/mm/numa.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index d9ab9da85eab..4e85564ef62a 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c

@@ -1697,6 +1697,23 @@ static const struct proc_ops topology_proc_ops = {

 int cpu_to_coregroup_id(int cpu)
 {
+	__be32 associativity[VPHN_ASSOC_BUFSIZE] = {0};
+	int index;
+

It would be good to have an assert here to ensure that we are calling
this function only when coregroups are enabled.

Else, we may end up returning the penultimate index which maps to the
chip-id.

We have a check below exactly for the same reason. Please look below.

quoted

+	if (cpu < 0 || cpu > nr_cpu_ids)
+		return -1;
+
+	if (!firmware_has_feature(FW_FEATURE_VPHN))
+		goto out;
+
+	if (vphn_get_associativity(cpu, associativity))
+		goto out;
+
+	index = of_read_number(associativity, 1);
+	if ((index > min_common_depth + 1) && coregroup_enabled)
+		return of_read_number(&associativity[index - 1], 1);

See ^above.

index would be the all the domains in the associativity array, 
min_common_depth would be where the primary domain or the chip-id is
defined. So we are reading the penultimate domain if and only if the
min_common_depth isn't the primary domain aka chip-id. 

What other check /assertions can we add?

quoted

+
+out:
 	return cpu_to_core_id(cpu);
 }

-- 
2.17.1

-- 
Thanks and Regards
Srikar Dronamraju