[PATCH 1/2] sched/psi: fix trigger window growth interpolation for large windows

[Tao Cui]

From: Tao Cui <cuitao@xxxxxxxxxx>

window_update() estimates the stall growth inside a partially elapsed
trigger window as

    growth += win->prev_growth * remaining / win->size

with `remaining = win->size - elapsed`.  The local `remaining` is a u32,
but win->size is a u64 and may be as large as WINDOW_MAX_US (10s = 10^10
ns), which exceeds UINT32_MAX (~4.29s).  For trigger windows larger than
~4.29s the assignment truncates `remaining`, the linear interpolation
collapses, and growth is heavily underestimated.

For a 10s window polled once per second with prev_growth equal to a full
window of stall, the estimate one second in is ~1.4s instead of 10s.  A
trigger configured as "some 5s 10s" then waits until the actual stall
reaches the threshold (~4s in) instead of firing as soon as the run-rate
predicts it (~1s in): a multi-second delay on the first window of a
stall.

Use a u64 `remaining` and mul_u64_u64_div_u64() for the product, which
also avoids the u64 overflow of `prev_growth * remaining` under heavy
pressure.  This runs in update_triggers() at trigger-evaluation rate (at
most a few times per second), not the scheduler hot path, so the helper's
cost is immaterial.

Fixes: 0e94682b73bf ("psi: introduce psi monitor")
Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
---
 kernel/sched/psi.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 4e152410653d..6514b44222cf 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -451,10 +451,11 @@ 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