linux-kernel - Re: [PATCH v2 1/2] iopoll: Call cpu

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Message-ID: <20230511064839.GG14287@atomide.com>
Date:   Thu, 11 May 2023 09:48:39 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Stephen Boyd <sboyd@...nel.org>,
        Tomasz Figa <tomasz.figa@...il.com>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Dejin Zheng <zhengdejin5@...il.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Russell King <linux@...linux.org.uk>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Tero Kristo <tero.kristo@...ux.intel.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-renesas-soc@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops

* Geert Uytterhoeven <geert+renesas@...der.be> [230510 13:23]:
> 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.

Reviewed-by: Tony Lindgren <tony@...mide.com>