[PATCH 2/6] sched/numa: fix scan-period ratio denominator checks

["Li Zhe" <lizhe.67@bytedance.com>]

update_task_scan_period() scales the scan period with two ratios:
local/(local + remote) and private/(private + shared).

The existing guard only checks local + shared. That normally implies
that both ratio denominators are non-zero because the locality and type
counters are updated from the same hint faults. Memory hotplug can make
that assumption fragile because task_numa_placement() folds the
private/shared buffers by walking online nodes, while the locality counters
were already charged when the fault was recorded.

The divide-by-zero was reproduced on v7.3-rc1 in a QEMU guest by
hot-removing a NUMA node after NUMA balancing had collected fault feedback.
The next NUMA hint fault enters task_numa_placement() with locality
feedback that was charged before node1 was removed. The per-node
private/shared buffers for node1 are skipped by for_each_online_node(), so
update_task_scan_period() can see local > 0 while private and shared are
both 0. The old local + shared guard lets that state pass, and the
subsequent private/(private + shared) calculation divides by zero.

Keep the slow-scan fallback for empty or unusable samples, but test the
actual denominators before calculating the ratios.

Fixes: 37ec97deb3a8 ("sched/numa: Slow down scan rate if shared faults dominate")
Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
 kernel/sched/fair.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1053437933ee..76532b8202dc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3698,6 +3698,8 @@ static void update_task_scan_period(struct task_struct *p,
 
 	unsigned long remote = p->numa_faults_locality[0];
 	unsigned long local = p->numa_faults_locality[1];
+	unsigned long locality_faults = local + remote;
+	unsigned long type_faults = private + shared;
 
 	/*
 	 * If there were no record hinting faults then either the task is
@@ -3706,7 +3708,7 @@ static void update_task_scan_period(struct task_struct *p,
 	 * migration then it implies we are migrating too quickly or the local
 	 * node is overloaded. In either case, scan slower
 	 */
-	if (local + shared == 0 || p->numa_faults_locality[2]) {
+	if (!locality_faults || !type_faults || p->numa_faults_locality[2]) {
 		p->numa_scan_period = min(p->numa_scan_period_max,
 			p->numa_scan_period << 1);
 
@@ -3723,8 +3725,8 @@ static void update_task_scan_period(struct task_struct *p,
 	 *	 >= NUMA_PERIOD_THRESHOLD scan period increases (scan slower)
 	 */
 	period_slot = DIV_ROUND_UP(p->numa_scan_period, NUMA_PERIOD_SLOTS);
-	lr_ratio = (local * NUMA_PERIOD_SLOTS) / (local + remote);
-	ps_ratio = (private * NUMA_PERIOD_SLOTS) / (private + shared);
+	lr_ratio = (local * NUMA_PERIOD_SLOTS) / locality_faults;
+	ps_ratio = (private * NUMA_PERIOD_SLOTS) / type_faults;
 
 	if (ps_ratio >= NUMA_PERIOD_THRESHOLD) {
 		/*
-- 
2.20.1