From patchwork Fri Jun 2 08:50:36 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Geert Uytterhoeven X-Patchwork-Id: 13264898 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id EF85DC7EE2A for ; Fri, 2 Jun 2023 08:51:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Message-Id:Date:Subject:Cc:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=VGm6G8nVLhyTkmGtd6Y3M4VrFJBe9UiJsObcTc9sev0=; b=fALJMoxvELyfCE 2c4TRAisb2Yd7XTWoy9QRQiFh1zDHIkgBQM0mCo0ZdQxZ26Gq1NTvmddOQjeXl0tDMl5TokxKlMDT 153eBtBxQtOUXbSu32bMEG/rwCQtjtdbUYImgUxYGxP216v56ETX9nA9EBMWJPNV4VimdMJJQjLKH 0z1SIfV0eo2wwpIuj6fpEWZYfOlc07A2NWfTrm2GMZHGJmZHqRHw7x46iIUIeGh59sd0U32GGVCGE qii61DjSW4LUjGQuG9RPOX7nlQJ5Gp0xk43AjE+cIw2HqVOFIxiQkfswbXNi0hW2M5ffZmr9Gex8d VZZGhpRm5OvC1dzVSsxA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1q50VN-006EMj-0Q; Fri, 02 Jun 2023 08:51:29 +0000 Received: from gauss.telenet-ops.be ([2a02:1800:120:4::f00:11]) by bombadil.infradead.org with esmtps (Exim 4.96 #2 (Red Hat Linux)) id 1q50VD-006EDh-1e for linux-arm-kernel@lists.infradead.org; Fri, 02 Jun 2023 08:51:23 +0000 Received: from laurent.telenet-ops.be (laurent.telenet-ops.be [IPv6:2a02:1800:110:4::f00:19]) by gauss.telenet-ops.be (Postfix) with ESMTPS id 4QXcDM43MGz4x2Kp for ; Fri, 2 Jun 2023 10:51:15 +0200 (CEST) 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 ) 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 ) id 1q50Uh-00APxS-V7; Fri, 02 Jun 2023 10:50:47 +0200 From: Geert Uytterhoeven To: Michael Turquette , Stephen Boyd , Yoshihiro Shimoda , Magnus Damm , Joerg Roedel , Robin Murphy Cc: Tomasz Figa , Sylwester Nawrocki , Will Deacon , Arnd Bergmann , Wolfram Sang , Dejin Zheng , Kai-Heng Feng , Nicholas Piggin , Heiko Carstens , Peter Zijlstra , Russell King , John Stultz , Thomas Gleixner , Tony Lindgren , Krzysztof Kozlowski , Tero Kristo , Ulf Hansson , "Rafael J . Wysocki" , Vincent Guittot , 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 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: References: MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230602_015119_692234_63AC4492 X-CRM114-Status: GOOD ( 18.74 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 Acked-by: Peter Zijlstra (Intel) Acked-by: Arnd Bergmann Reviewed-by: Tony Lindgren Reviewed-by: Ulf Hansson --- 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; \ })