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

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Message-ID: <515a4c94ce764c58ab9a311d6cc5187f@AcuMS.aculab.com>
Date:   Tue, 23 May 2023 08:55:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Geert Uytterhoeven' <geert@...ux-m68k.org>
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

From: Geert Uytterhoeven
> Sent: 23 May 2023 08:30
> 
> 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.

Hmmm.... tens of milliseconds is really much too long for RT threads.
A limit nearer 1ms would be barely acceptable.

	David

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