linux-kernel - Re: [RFC PATCH] timers: Optimize usleep

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Date:   Mon, 23 May 2022 09:47:23 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     dan.carpenter@...cle.com, Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [RFC PATCH] timers: Optimize usleep_range()

On Sun, May 22, 2022 at 9:38 AM Christophe JAILLET
<christophe.jaillet@...adoo.fr> wrote:
> diff --git a/include/linux/delay.h b/include/linux/delay.h
> index 039e7e0c7378..e84e7f9c1a47 100644
> --- a/include/linux/delay.h
> +++ b/include/linux/delay.h
> @@ -61,10 +61,23 @@ void msleep(unsigned int msecs);
>  unsigned long msleep_interruptible(unsigned int msecs);
>  void usleep_range_state(unsigned long min, unsigned long max,
>                         unsigned int state);
> +void __nsleep_range_delta_state(u64 min, u64 delta, unsigned int state);
>
>  static inline void usleep_range(unsigned long min, unsigned long max)
>  {
> -       usleep_range_state(min, max, TASK_UNINTERRUPTIBLE);
> +       /*
> +        * Most of the time min and max are constant, so the time delta and the
> +        * convertion to ns can be computed at compile time.
> +        */
> +       if (__builtin_constant_p(min) &&
> +           __builtin_constant_p(max)) {
> +               u64 delta = (u64)(max - min) * NSEC_PER_USEC;
> +
> +               __nsleep_range_delta_state(min * NSEC_PER_USEC, delta,
> +                                          TASK_UNINTERRUPTIBLE);
> +       } else {
> +               usleep_range_state(min, max, TASK_UNINTERRUPTIBLE);
> +       }
>  }

It seems the key optimization is just moving the multiplication to the
inlined header function, so rather than duplicating most of
usleep_range_state() in __nsleep_range_delta_state(), could you not
switch all uses of usleep_range_state() to use the
__nsleep_range_delta_state()?  Then you don't need the
__buildint_constant_p() checks since the compiler can decide on its
own, no?


> +/**
> + * __nsleep_range_delta_state - Sleep for an approximate time in a given state
> + * @min:       Minimum time in nsecs to sleep
> + * @delta:     Maximum time in nsecs to sleep

I don't think that's the right value description here. Maybe this
should be the slack value?


thanks
-john