Subject: Re: [PATCH v5 05/10] powerpc/smp: Dont assume l2-cache to be superset of sibling

From: Srikar Dronamraju <[email-masked]>
Date: Sun, 13 Sep 2020 22:40:38 +0530

Fix to make it work where CPUs dont have a l2-cache element.

------------>8-----------------------------------------8<---------------------

>From b25d47b01b7195b1df19083a4043fa6a87a901a3 Mon Sep 17 00:00:00 2001
From: Srikar Dronamraju <[email-masked]>
Date: Thu, 9 Jul 2020 13:33:38 +0530
Subject: [PATCH v5.2 05/10] powerpc/smp: Dont assume l2-cache to be superset of
 sibling

Current code assumes that cpumask of cpus sharing a l2-cache mask will
always be a superset of cpu_sibling_mask.

Lets stop that assumption. cpu_l2_cache_mask is a superset of
cpu_sibling_mask if and only if shared_caches is set.

Cc: linuxppc-dev <[email-masked]>
Cc: LKML <[email-masked]>
Cc: Michael Ellerman <[email-masked]>
Cc: Nicholas Piggin <[email-masked]>
Cc: Anton Blanchard <[email-masked]>
Cc: Oliver O'Halloran <[email-masked]>
Cc: Nathan Lynch <[email-masked]>
Cc: Michael Neuling <[email-masked]>
Cc: Gautham R Shenoy <[email-masked]>
Cc: Ingo Molnar <[email-masked]>
Cc: Peter Zijlstra <[email-masked]>
Cc: Valentin Schneider <[email-masked]>
Cc: Jordan Niethe <[email-masked]>
Cc: Vaidyanathan Srinivasan <[email-masked]>
Reviewed-by: Gautham R. Shenoy <[email-masked]>
Signed-off-by: Srikar Dronamraju <[email-masked]>
---
Changelog v1 -> v2:
	Set cpumask after verifying l2-cache. (Gautham)

Changelog v5 -> v5.2:
	If cpu has no l2-cache set cpumask as per its
                 sibling mask. (Michael Ellerman)

 arch/powerpc/kernel/smp.c | 43 +++++++++++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 9f4333d..168532e 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1186,9 +1186,23 @@ static bool update_mask_by_l2(int cpu, struct cpumask *(*mask_fn)(int))
 	int i;
 
 	l2_cache = cpu_to_l2cache(cpu);
-	if (!l2_cache)
+	if (!l2_cache) {
+		struct cpumask *(*sibling_mask)(int) = cpu_sibling_mask;
+
+		/*
+		 * If no l2cache for this CPU, assume all siblings to share
+		 * cache with this CPU.
+		 */
+		if (has_big_cores)
+			sibling_mask = cpu_smallcore_mask;
+
+		for_each_cpu(i, sibling_mask(cpu))
+			set_cpus_related(cpu, i, cpu_l2_cache_mask);
+
 		return false;
+	}
 
+	cpumask_set_cpu(cpu, mask_fn(cpu));
 	for_each_cpu(i, cpu_online_mask) {
 		/*
 		 * when updating the marks the current CPU has not been marked
@@ -1271,29 +1285,30 @@ static void add_cpu_to_masks(int cpu)
 	 * add it to it's own thread sibling mask.
 	 */
 	cpumask_set_cpu(cpu, cpu_sibling_mask(cpu));
+	cpumask_set_cpu(cpu, cpu_core_mask(cpu));
 
 	for (i = first_thread; i < first_thread + threads_per_core; i++)
 		if (cpu_online(i))
 			set_cpus_related(i, cpu, cpu_sibling_mask);
 
 	add_cpu_to_smallcore_masks(cpu);
-	/*
-	 * Copy the thread sibling mask into the cache sibling mask
-	 * and mark any CPUs that share an L2 with this CPU.
-	 */
-	for_each_cpu(i, cpu_sibling_mask(cpu))
-		set_cpus_related(cpu, i, cpu_l2_cache_mask);
 	update_mask_by_l2(cpu, cpu_l2_cache_mask);
 
-	/*
-	 * Copy the cache sibling mask into core sibling mask and mark
-	 * any CPUs on the same chip as this CPU.
-	 */
-	for_each_cpu(i, cpu_l2_cache_mask(cpu))
-		set_cpus_related(cpu, i, cpu_core_mask);
+	if (pkg_id == -1) {
+		struct cpumask *(*mask)(int) = cpu_sibling_mask;
+
+		/*
+		 * Copy the sibling mask into core sibling mask and
+		 * mark any CPUs on the same chip as this CPU.
+		 */
+		if (shared_caches)
+			mask = cpu_l2_cache_mask;
+
+		for_each_cpu(i, mask(cpu))
+			set_cpus_related(cpu, i, cpu_core_mask);
 
-	if (pkg_id == -1)
 		return;
+	}
 
 	for_each_cpu(i, cpu_online_mask)
 		if (get_physical_package_id(i) == pkg_id)
-- 
2.17.1