[PATCH 0/2] sched/psi: Fix trigger time arithmetic

[PATCH 0/2] sched/psi: Fix trigger time arithmetic

[Guopeng Zhang]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

PSI trigger time handling has two integer-width bugs. Long windows can
be truncated during growth interpolation, and the multiplication used
for the interpolation can overflow before the division. Separately,
the microsecond-to-nanosecond conversion can wrap on 32-bit systems
before the result is stored in a u64.

Fix the two issues independently.

Guopeng Zhang (2):
  sched/psi: Fix long-window growth interpolation
  sched/psi: Fix overflow in trigger time conversion on 32-bit

 kernel/sched/psi.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)


base-commit: 1a1757b76427f6201bfe0bf1bea9f7574f332a93
-- 
2.43.0

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

[Guopeng Zhang]

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

[PATCH 2/2] sched/psi: Fix overflow in trigger time conversion on 32-bit

[Guopeng Zhang]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

threshold_us and window_us are u32, while NSEC_PER_USEC is 1000L. On
32-bit architectures, multiplication by NSEC_PER_USEC is evaluated
using 32-bit unsigned arithmetic and can wrap before the result is
assigned to the u64 trigger fields.

For a valid 4,000,000 us threshold and 6,000,000 us window, the
threshold is stored as 4,000,000,000 ns, while the window wraps to
1,705,032,704 ns. The stored window is therefore shorter than the
threshold, so the trigger no longer monitors the requested 4-second
threshold over a 6-second window.

Cast both values to u64 before the multiplication so that the
conversion to nanoseconds is performed using 64-bit arithmetic.

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

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 8e4df8b17c25..f0a5976f49dc 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1391,8 +1391,8 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group, char *buf,
 
 	t->group = group;
 	t->state = state;
-	t->threshold = threshold_us * NSEC_PER_USEC;
-	t->win.size = window_us * NSEC_PER_USEC;
+	t->threshold = (u64)threshold_us * NSEC_PER_USEC;
+	t->win.size = (u64)window_us * NSEC_PER_USEC;
 	window_reset(&t->win, sched_clock(),
 			group->total[PSI_POLL][t->state], 0);
 
-- 
2.43.0