Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Tony Lindgren' <tony@atomide.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>
CC:     Stephen Boyd <sboyd@kernel.org>,
        Tomasz Figa <tomasz.figa@gmail.com>,
        Sylwester Nawrocki <s.nawrocki@samsung.com>,
        Will Deacon <will@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Wolfram Sang <wsa+renesas@sang-engineering.com>,
        Dejin Zheng <zhengdejin5@gmail.com>,
        Kai-Heng Feng <kai.heng.feng@canonical.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Russell King <linux@armlinux.org.uk>,
        John Stultz <jstultz@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Krzysztof Kozlowski" <krzk@kernel.org>,
        Tero Kristo <tero.kristo@linux.intel.com>,
        "Ulf Hansson" <ulf.hansson@linaro.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-renesas-soc@vger.kernel.org" 
        <linux-renesas-soc@vger.kernel.org>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops
Thread-Topic: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops
Thread-Index: AQHZg9SqgvUK/klkyEy9n2eMPFqMgK9U5DUg
Date:   Thu, 11 May 2023 10:48:37 +0000
Message-ID: <494a000774b546e4aae00ae0a7dfeae4@AcuMS.aculab.com>
References: <cover.1683722688.git.geert+renesas@glider.be>
 <fe235a1f65bb6c86d2afcdf52d85f80ae728dcc5.1683722688.git.geert+renesas@glider.be>
 <20230511064839.GG14287@atomide.com>
In-Reply-To: <20230511064839.GG14287@atomide.com>
Accept-Language: en-GB, en-US
MIME-Version: 1.0
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Precedence: bulk

From: Tony Lindgren
> Sent: 11 May 2023 07:49
> 
> * Geert Uytterhoeven <geert+renesas@glider.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.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)