powerpc/powernv: Respect max_cpus while initializing core split/unsplit feature.

In kdump kernel we see a hang during subcore_init() at
unsplit_core()->wait_for_sync_step(). In kdump kernel we always boot with
maxcpus=1 and all other cpus are waiting inside OPAL, hence with 1 online
cpu the master thread keep waiting on secondary threads to set split_state
indefinitely. This is even true for all cases where max_cpus is not aligned
with threads_per_core. This patch fixes this issue by disabling
core split/unsplit feature if max_cpus are not aligned with threads_per_core.
This also fixes kdump hang issue.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/arch/powerpc/platforms/powernv/subcore.c b/arch/powerpc/platforms/powernv/subcore.c
index 75d0624673c10..1728ed8f28706 100644
--- a/arch/powerpc/platforms/powernv/subcore.c
+++ b/arch/powerpc/platforms/powernv/subcore.c
@@ -287,9 +287,14 @@ static int cpu_update_split_mode(void *data)
 
 	if (this_cpu_ptr(&split_state)->master) {
 		/* Wait for all cpus to finish before we touch subcores_per_core */
-		for_each_present_cpu(cpu)
+		for_each_present_cpu(cpu) {
+			/* Check if we reached maxcpus */
+			if (cpu >= setup_max_cpus)
+				break;
+
 			while(per_cpu(split_state, cpu).step < SYNC_STEP_FINISHED)
 				barrier();
+		}
 
 		new_split_mode = 0;
 
@@ -390,6 +395,13 @@ static int subcore_init(void)
 	if (!cpu_has_feature(CPU_FTR_ARCH_207S))
 		return 0;
 
+	/*
+	 * Respect max_cpus kernel parameter.
+	 * Continue only if max_cpus are aligned to threads_per_core.
+	 */
+	if (setup_max_cpus % threads_per_core)
+		return 0;
+
 	BUG_ON(!alloc_cpumask_var(&cpu_offline_mask, GFP_KERNEL));
 
 	set_subcores_per_core(1);