[PATCH] sched/numa: Fix time unit mismatch in scan staggering

[Hui Su]

init_numa_balancing() stores node_stamp in nanoseconds, but
task_scan_max() and numa_scan_period are in milliseconds. The existing
min_t() compares task_scan_max() in milliseconds with a nanosecond
value, so it can initialize node_stamp far too small and defeat
staggering for new threads sharing an mm.

For example, with the default scan periods and two mm users, the
intended stagger is 2000 ms. The existing code instead computes a
60000 ns delay; after adding the two-tick offset, node_stamp is about
2.06 ms with HZ=1000.

Moreover, min_t() and delay use unsigned int, so nanosecond values above
UINT_MAX are truncated. On 32-bit systems, the multiplication can also
overflow before the value is converted.

Compare the delay in milliseconds using u64 and convert to nanoseconds
after the minimum is selected. This preserves the intended cap and keeps
node_stamp in the unit consumed by task_tick_numa().

This prevents NUMA balancing scans from being concentrated for
multithreaded workloads and avoids shortening the initial stagger
through integer truncation.

Fixes: 137844759843 ("sched/numa: Stagger NUMA balancing scan periods for new threads")
Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
 kernel/sched/fair.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6d881e530f89..6924cf3b3a67 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4413,12 +4413,11 @@ void init_numa_balancing(u64 clone_flags, struct task_struct *p)
 	 * already by arch_dup_task_struct but stagger when scans start.
 	 */
 	if (mm) {
-		unsigned int delay;
+		u64 delay_ms;
 
-		delay = min_t(unsigned int, task_scan_max(current),
-			current->numa_scan_period * mm_users * NSEC_PER_MSEC);
-		delay += 2 * TICK_NSEC;
-		p->node_stamp = delay;
+		delay_ms = (u64)current->numa_scan_period * mm_users;
+		delay_ms = min_t(u64, delay_ms, task_scan_max(current));
+		p->node_stamp = delay_ms * NSEC_PER_MSEC + 2 * TICK_NSEC;
 	}
 }
 
-- 
2.54.0