From patchwork Wed May 10 13:23:13 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Geert Uytterhoeven X-Patchwork-Id: 681094 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D3569C7EE2D for ; Wed, 10 May 2023 13:23:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237005AbjEJNXm (ORCPT ); Wed, 10 May 2023 09:23:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58802 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236792AbjEJNXk (ORCPT ); Wed, 10 May 2023 09:23:40 -0400 Received: from albert.telenet-ops.be (albert.telenet-ops.be [IPv6:2a02:1800:110:4::f00:1a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A147B5FC0 for ; Wed, 10 May 2023 06:23:35 -0700 (PDT) Received: from ramsan.of.borg ([IPv6:2a02:1810:ac12:ed30:cc75:d8ef:4074:8af9]) by albert.telenet-ops.be with bizsmtp id v1PG2900C3l7qvk061PGDS; Wed, 10 May 2023 15:23:32 +0200 Received: from rox.of.borg ([192.168.97.57]) by ramsan.of.borg with esmtp (Exim 4.95) (envelope-from ) id 1pwjme-001nDk-DC; Wed, 10 May 2023 15:23:16 +0200 Received: from geert by rox.of.borg with local (Exim 4.95) (envelope-from ) id 1pwjmm-00F6qK-40; Wed, 10 May 2023 15:23:16 +0200 From: Geert Uytterhoeven To: Stephen Boyd , 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 Cc: linux-arm-kernel@lists.infradead.org, linux-renesas-soc@vger.kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, Geert Uytterhoeven Subject: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops Date: Wed, 10 May 2023 15:23:13 +0200 Message-Id: X-Mailer: git-send-email 2.34.1 In-Reply-To: References: MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.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 --- 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; \ }) From patchwork Wed May 10 13:23:14 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Geert Uytterhoeven X-Patchwork-Id: 680806 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 10C5CC7EE22 for ; Wed, 10 May 2023 13:23:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236834AbjEJNXk (ORCPT ); Wed, 10 May 2023 09:23:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58784 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232419AbjEJNXj (ORCPT ); Wed, 10 May 2023 09:23:39 -0400 Received: from andre.telenet-ops.be (andre.telenet-ops.be [IPv6:2a02:1800:120:4::f00:15]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A04B75BBD for ; Wed, 10 May 2023 06:23:35 -0700 (PDT) Received: from ramsan.of.borg ([IPv6:2a02:1810:ac12:ed30:cc75:d8ef:4074:8af9]) by andre.telenet-ops.be with bizsmtp id v1PG2900T3l7qvk011PGsd; Wed, 10 May 2023 15:23:32 +0200 Received: from rox.of.borg ([192.168.97.57]) by ramsan.of.borg with esmtp (Exim 4.95) (envelope-from ) id 1pwjme-001nDl-DW; Wed, 10 May 2023 15:23:16 +0200 Received: from geert by rox.of.borg with local (Exim 4.95) (envelope-from ) id 1pwjmm-00F6qN-4j; Wed, 10 May 2023 15:23:16 +0200 From: Geert Uytterhoeven To: Stephen Boyd , 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 Cc: linux-arm-kernel@lists.infradead.org, linux-renesas-soc@vger.kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, Geert Uytterhoeven Subject: [PATCH v2 2/2] iopoll: Do not use timekeeping in read_poll_timeout_atomic() Date: Wed, 10 May 2023 15:23:14 +0200 Message-Id: <8db63020d18fc22e137e4a8f0aa15e6b9949a6f6.1683722688.git.geert+renesas@glider.be> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org read_poll_timeout_atomic() uses ktime_get() to implement the timeout feature, just like its non-atomic counterpart. However, there are several issues with this, due to its use in atomic contexts: 1. When called in the s2ram path (as typically done by clock or PM domain drivers), timekeeping may be suspended, triggering the WARN_ON(timekeeping_suspended) in ktime_get(): WARNING: CPU: 0 PID: 654 at kernel/time/timekeeping.c:843 ktime_get+0x28/0x78 Calling ktime_get_mono_fast_ns() instead of ktime_get() would get rid of that warning. However, that would break timeout handling, as (at least on systems with an ARM architectured timer), the time returned by ktime_get_mono_fast_ns() does not advance while timekeeping is suspended. Interestingly, (on the same ARM systems) the time returned by ktime_get() does advance while timekeeping is suspended, despite the warning. 2. Depending on the actual clock source, and especially before a high-resolution clocksource (e.g. the ARM architectured timer) becomes available, time may not advance in atomic contexts, thus breaking timeout handling. Fix this by abandoning the idea that one can rely on timekeeping to implement timeout handling in all atomic contexts, and switch from a global time-based to a locally-estimated timeout handling. In most (all?) cases the timeout condition is exceptional and an error condition, hence any additional delays due to underestimating wall clock time are irrelevant. Signed-off-by: Geert Uytterhoeven Acked-by: Arnd Bergmann Reviewed-by: Tony Lindgren Reviewed-by: Ulf Hansson --- Alternatively, one could use a mixed approach (use both ktime_get_mono_fast_ns() and a local (under)estimate, and timeout on the earliest occasion), but I think that would complicate things without much gain. v2: - New. --- include/linux/iopoll.h | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) diff --git a/include/linux/iopoll.h b/include/linux/iopoll.h index 0417360a6db9b0d6..bb2e1d9117e96679 100644 --- a/include/linux/iopoll.h +++ b/include/linux/iopoll.h @@ -81,22 +81,30 @@ delay_before_read, args...) \ ({ \ u64 __timeout_us = (timeout_us); \ + s64 __left_ns = __timeout_us * NSEC_PER_USEC; \ unsigned long __delay_us = (delay_us); \ - ktime_t __timeout = ktime_add_us(ktime_get(), __timeout_us); \ - if (delay_before_read && __delay_us) \ + u64 __delay_ns = __delay_us * NSEC_PER_USEC; \ + if (delay_before_read && __delay_us) { \ udelay(__delay_us); \ + if (__timeout_us) \ + __left_ns -= __delay_ns; \ + } \ for (;;) { \ (val) = op(args); \ if (cond) \ break; \ - if (__timeout_us && \ - ktime_compare(ktime_get(), __timeout) > 0) { \ + if (__timeout_us && __left_ns < 0) { \ (val) = op(args); \ break; \ } \ - if (__delay_us) \ + if (__delay_us) { \ udelay(__delay_us); \ + if (__timeout_us) \ + __left_ns -= __delay_ns; \ + } \ cpu_relax(); \ + if (__timeout_us) \ + __left_ns--; \ } \ (cond) ? 0 : -ETIMEDOUT; \ })