linux-kernel - Re: [PATCH] cpuidle: idle exit

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Date:   Mon, 11 Dec 2023 21:52:31 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bo Ye <bo.ye@...iatek.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        yongdong.zhang@...iatek.com, mtk24676 <C.Cheng@...iatek.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] cpuidle: idle exit_latency overflow

On Thu, Nov 16, 2023 at 2:26 PM Bo Ye <bo.ye@...iatek.com> wrote:
>
> From: mtk24676 <C.Cheng@...iatek.com>
>
> In detail:
> In kernel-6.1, in the __cpuidle_driver_init function in
> driver/cpuidle/driver.c, there is a line of code that causes
> an overflow. The line is s->exit_latency_ns = s->exit_latency
> * NSEC_PER_USEC. The overflow occurs because the product of an
> int type and a constant exceeds the range of the int type.

In general, but does it actually occur in that code?  IOW, is there
any system for which the exit latency of an idle state is so large
that it will trigger the overflow, for example?

> In C language, when you perform a multiplication operation, if
> both operands are of int type, the multiplication operation is
> performed on the int type, and then the result is converted to
> the target type.

Right, that's how C works.

> This means that if the product of int type
> multiplication exceeds the range that int type can represent,
> an overflow will occur even if you store the result in a
> variable of int64_t type.

True.  However, does this really happen in the particular case at hand?

If not, it would be better to say something like "For a multiplication
of two int values, it is better to use mul_u32_u32() that prevents
overflows from occurring."

> Signed-off-by: mtk24676 <C.Cheng@...iatek.com>
> Signed-off-by: bo.ye <bo.ye@...iatek.com>
> ---
>
> diff --git a/drivers/cpuidle/driver.c b/drivers/cpuidle/driver.c
> index d9cda7f..631ca16 100644
> --- a/drivers/cpuidle/driver.c
> +++ b/drivers/cpuidle/driver.c
> @@ -187,7 +187,7 @@
>                         s->target_residency = div_u64(s->target_residency_ns, NSEC_PER_USEC);
>
>                 if (s->exit_latency > 0)
> -                       s->exit_latency_ns = s->exit_latency * NSEC_PER_USEC;
> +                       s->exit_latency_ns = (u64)s->exit_latency * NSEC_PER_USEC;

mul_u32_u32()/?

>                 else if (s->exit_latency_ns < 0)
>                         s->exit_latency_ns =  0;
>                 else