[PATCH 1/2] sched/psi: Fix long-window growth interpolation

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

PSI trigger windows are stored in nanoseconds and can be up to 10
seconds, but window_update() stores the remaining interval in a u32.
For example, after 2 seconds have elapsed in a 10-second window, the
remaining 8,000,000,000 ns is truncated to 3,705,032,704 ns.

Making remaining a u64 avoids the truncation, but the multiplication
can still overflow before the division. Both win->prev_growth and
remaining can be close to 10,000,000,000, so their product can exceed
U64_MAX.

Store the remaining interval in a u64 and use
mul_u64_u64_div_u64() to calculate the interpolation without
overflowing the intermediate product.

Fixes: 0e94682b73bf ("psi: introduce psi monitor")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 kernel/sched/psi.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 4e152410653d..8e4df8b17c25 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -137,6 +137,7 @@
  * sampling of the aggregate task states would be.
  */
 #include <linux/sched/clock.h>
+#include <linux/math64.h>
 #include <linux/workqueue.h>
 #include <linux/psi.h>
 #include "sched.h"
@@ -451,10 +452,11 @@ static u64 window_update(struct psi_window *win, u64 now, u64 value)
 	if (elapsed > win->size)
 		window_reset(win, now, value, growth);
 	else {
-		u32 remaining;
+		u64 remaining;
 
 		remaining = win->size - elapsed;
-		growth += div64_u64(win->prev_growth * remaining, win->size);
+		growth += mul_u64_u64_div_u64(win->prev_growth, remaining,
+					      win->size);
 	}
 
 	return growth;
-- 
2.43.0