Re: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops

[Ulf Hansson]

On Wed, 10 May 2023 at 15:23, Geert Uytterhoeven
<geert+renesas@xxxxxxxxx> wrote:
>
> It is considered good practice to call cpu_relax() in busy loops, see
> Documentation/process/volatile-considered-harmful.rst.  This can not
> only lower CPU power consumption or yield to a hyperthreaded twin
> processor, but also allows an architecture to mitigate hardware issues
> (e.g. ARM Erratum 754327 for Cortex-A9 prior to r2p0) in the
> architecture-specific cpu_relax() implementation.
>
> In addition, cpu_relax() is also a compiler barrier.  It is not
> immediately obvious that the @op argument "function" will result in an
> actual function call (e.g. in case of inlining).
>
> Where a function call is a C sequence point, this is lost on inlining.
> Therefore, with agressive enough optimization it might be possible for
> the compiler to hoist the:
>
>         (val) = op(args);
>
> "load" out of the loop because it doesn't see the value changing. The
> addition of cpu_relax() would inhibit this.
>
> As the iopoll helpers lack calls to cpu_relax(), people are sometimes
> reluctant to use them, and may fall back to open-coded polling loops
> (including cpu_relax() calls) instead.
>
> Fix this by adding calls to cpu_relax() to the iopoll helpers:
>   - For the non-atomic case, it is sufficient to call cpu_relax() in
>     case of a zero sleep-between-reads value, as a call to
>     usleep_range() is a safe barrier otherwise.  However, it doesn't
>     hurt to add the call regardless, for simplicity, and for similarity
>     with the atomic case below.
>   - For the atomic case, cpu_relax() must be called regardless of the
>     sleep-between-reads value, as there is no guarantee all
>     architecture-specific implementations of udelay() handle this.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Acked-by: Arnd Bergmann <arnd@xxxxxxxx>

Makes sense to me! Feel free to add:

Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>

Kind regards
Uffe

> ---
> v2:
>   - Add Acked-by,
>   - Add compiler barrier and inlining explanation (thanks, Peter!).
> ---
>  include/linux/iopoll.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/include/linux/iopoll.h b/include/linux/iopoll.h
> index 2c8860e406bd8cae..0417360a6db9b0d6 100644
> --- a/include/linux/iopoll.h
> +++ b/include/linux/iopoll.h
> @@ -53,6 +53,7 @@
>                 } \
>                 if (__sleep_us) \
>                         usleep_range((__sleep_us >> 2) + 1, __sleep_us); \
> +               cpu_relax(); \
>         } \
>         (cond) ? 0 : -ETIMEDOUT; \
>  })
> @@ -95,6 +96,7 @@
>                 } \
>                 if (__delay_us) \
>                         udelay(__delay_us); \
> +               cpu_relax(); \
>         } \
>         (cond) ? 0 : -ETIMEDOUT; \
>  })
> --
> 2.34.1
>