Message ID | 45c87bec3397fdd704376807f0eec5cc71be440f.1685692810.git.geert+renesas@glider.be |
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State | New |
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[2620:137:e000::1:20]) by mx.google.com with ESMTP id bs8-20020a632808000000b0053f21272753si695406pgb.9.2023.06.02.02.05.07; Fri, 02 Jun 2023 02:05:20 -0700 (PDT) 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; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235023AbjFBIwq (ORCPT <rfc822;limurcpp@gmail.com> + 99 others); Fri, 2 Jun 2023 04:52:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51054 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234919AbjFBIwA (ORCPT <rfc822;linux-kernel@vger.kernel.org>); Fri, 2 Jun 2023 04:52:00 -0400 Received: from laurent.telenet-ops.be (laurent.telenet-ops.be [IPv6:2a02:1800:110:4::f00:19]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3A5DD1735 for <linux-kernel@vger.kernel.org>; Fri, 2 Jun 2023 01:51:16 -0700 (PDT) Received: from ramsan.of.borg ([IPv6:2a02:1810:ac12:ed30:158c:2ccf:1f70:e136]) by laurent.telenet-ops.be with bizsmtp id 48qo2A00B1tRZS8018qoyS; Fri, 02 Jun 2023 10:51:14 +0200 Received: from rox.of.borg ([192.168.97.57]) by ramsan.of.borg with esmtp (Exim 4.95) (envelope-from <geert@linux-m68k.org>) id 1q50UO-00BhY9-RW; Fri, 02 Jun 2023 10:50:48 +0200 Received: from geert by rox.of.borg with local (Exim 4.95) (envelope-from <geert@linux-m68k.org>) id 1q50Uh-00APxS-V7; Fri, 02 Jun 2023 10:50:47 +0200 From: Geert Uytterhoeven <geert+renesas@glider.be> To: Michael Turquette <mturquette@baylibre.com>, Stephen Boyd <sboyd@kernel.org>, Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>, Magnus Damm <magnus.damm@gmail.com>, Joerg Roedel <joro@8bytes.org>, Robin Murphy <robin.murphy@arm.com> Cc: 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>, Tony Lindgren <tony@atomide.com>, 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-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-renesas-soc@vger.kernel.org, linux-pm@vger.kernel.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, Geert Uytterhoeven <geert+renesas@glider.be> Subject: [PATCH v3 1/7] iopoll: Call cpu_relax() in busy loops Date: Fri, 2 Jun 2023 10:50:36 +0200 Message-Id: <45c87bec3397fdd704376807f0eec5cc71be440f.1685692810.git.geert+renesas@glider.be> X-Mailer: git-send-email 2.34.1 In-Reply-To: <cover.1685692810.git.geert+renesas@glider.be> References: <cover.1685692810.git.geert+renesas@glider.be> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-2.4 required=5.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_LOW,SPF_HELO_NONE,SPF_NONE, T_SCC_BODY_TEXT_LINE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: <linux-kernel.vger.kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1767581123738673990?= X-GMAIL-MSGID: =?utf-8?q?1767581123738673990?= |
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iopoll: Busy loop and timeout improvements + conversions
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Commit Message
Geert Uytterhoeven
June 2, 2023, 8:50 a.m. UTC
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@glider.be> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Arnd Bergmann <arnd@arndb.de> Reviewed-by: Tony Lindgren <tony@atomide.com> Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org> --- v3: - Add Reviewed-by, 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; \ })