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

[Suren Baghdasaryan]

On Fri, Jul 17, 2026 at 3:29 AM Guopeng Zhang <guopeng.zhang@xxxxxxxxx> wrote:
>
> From: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>

Sorry for the delay in reviewing these fixes. The fix looks correct to me.

Acked-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
>  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