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

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Date:   Tue, 23 May 2023 09:29:53 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Tony Lindgren <tony@...mide.com>, 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-arm-kernel@...ts.infradead.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops

Hi David,

On Thu, May 11, 2023 at 12:49 PM David Laight <David.Laight@...lab.com> wrote:
> > * 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.
>
> Don't you also need to call cond_resched() (at least some times).
> Otherwise the process can't be pre-empted and a RT process
> that last ran on that cpu will never be scheduled.

According to [1], cond_resched() must be called at least once per few
tens of milliseconds.

read_poll_timeout() uses usleep_range(), which calls schedule_hrtimeout_range().
read_poll_timeout_atomic() should not be used with multi-ms timeouts anyway.

So I guess we're OK?

[1] https://elixir.bootlin.com/linux/latest/source/Documentation/RCU/Design/Requirements/Requirements.rst#L2348

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds