From nobody Tue Dec 30 13:27:22 2025 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 4C146C47072 for ; Wed, 15 Nov 2023 09:04:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234698AbjKOJE4 (ORCPT ); Wed, 15 Nov 2023 04:04:56 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41766 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234675AbjKOJEy (ORCPT ); Wed, 15 Nov 2023 04:04:54 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 38384116; Wed, 15 Nov 2023 01:04:51 -0800 (PST) Date: Wed, 15 Nov 2023 09:04:48 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1700039089; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CiagETb+EIhLnlRlKgxN+Qn60M9WIRvjphEzuDjgzB8=; b=2hJwdjws3Kr+gjDuD09s+jOlHs9Rp9CgB78iRIo0YqAXdNQ6ONvV9BjY7BT6dL2eGcVIQI NtBM6WYo34VXG9v5MerOuHd5qY13M4R/hD0Ui7/Mv40g+85S0pAMXef8Rd5qb3C334e8L4 CVuvqQbRnxD6Hdc3pSwTkaXiGEalXx7mndov7ojGGh78hOEIwUZhfOd1MEbxrC8qhmRGWq Abd986crt8YtZ+mtMfbvxnpZa5VDI8vS0T4AqhXHmc1Uw8pjytbqR2Kk+Yq7KbDotsih3j EC7wo4a2FK8R2iboMydLfv37nELV4CXs/ZhbMJ9kcNP3SzmP1t/Z+mjZoK7m0g== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1700039089; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CiagETb+EIhLnlRlKgxN+Qn60M9WIRvjphEzuDjgzB8=; b=L0A82ts+pJSxc8152bNLSc3s6kkwShfbTfhTmkwTDYRiHwmabTrMfFmMBfRGANbhj6dc5+ nWZKeOEJv4gtfHDw== From: "tip-bot2 for Frederic Weisbecker" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: sched/core] sched/cpuidle: Comment about timers requirements VS idle handler Cc: Frederic Weisbecker , "Peter Zijlstra (Intel)" , "Rafael J. Wysocki" , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20231114193840.4041-2-frederic@kernel.org> References: <20231114193840.4041-2-frederic@kernel.org> MIME-Version: 1.0 Message-ID: <170003908886.391.2109617605485917006.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the sched/core branch of tip: Commit-ID: dd5403869a40595eb953f12e8cd2bb57bb88bb67 Gitweb: https://git.kernel.org/tip/dd5403869a40595eb953f12e8cd2bb57b= b88bb67 Author: Frederic Weisbecker AuthorDate: Tue, 14 Nov 2023 14:38:39 -05:00 Committer: Peter Zijlstra CommitterDate: Wed, 15 Nov 2023 09:57:51 +01:00 sched/cpuidle: Comment about timers requirements VS idle handler Add missing explanation concerning IRQs re-enablement constraints in the cpuidle path against timers. Signed-off-by: Frederic Weisbecker Signed-off-by: Peter Zijlstra (Intel) Acked-by: Rafael J. Wysocki Link: https://lkml.kernel.org/r/20231114193840.4041-2-frederic@kernel.org --- kernel/sched/idle.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index 565f837..3123192 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -258,6 +258,36 @@ static void do_idle(void) while (!need_resched()) { rmb(); =20 + /* + * Interrupts shouldn't be re-enabled from that point on until + * the CPU sleeping instruction is reached. Otherwise an interrupt + * may fire and queue a timer that would be ignored until the CPU + * wakes from the sleeping instruction. And testing need_resched() + * doesn't tell about pending needed timer reprogram. + * + * Several cases to consider: + * + * - SLEEP-UNTIL-PENDING-INTERRUPT based instructions such as + * "wfi" or "mwait" are fine because they can be entered with + * interrupt disabled. + * + * - sti;mwait() couple is fine because the interrupts are + * re-enabled only upon the execution of mwait, leaving no gap + * in-between. + * + * - ROLLBACK based idle handlers with the sleeping instruction + * called with interrupts enabled are NOT fine. In this scheme + * when the interrupt detects it has interrupted an idle handler, + * it rolls back to its beginning which performs the + * need_resched() check before re-executing the sleeping + * instruction. This can leak a pending needed timer reprogram. + * If such a scheme is really mandatory due to the lack of an + * appropriate CPU sleeping instruction, then a FAST-FORWARD + * must instead be applied: when the interrupt detects it has + * interrupted an idle handler, it must resume to the end of + * this idle handler so that the generic idle loop is iterated + * again to reprogram the tick. + */ local_irq_disable(); =20 if (cpu_is_offline(cpu)) {