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[2620:137:e000::1:20]) by mx.google.com with ESMTP id f20-20020a633814000000b00553813c2df0si5426834pga.513.2023.06.18.13.54.13; Sun, 18 Jun 2023 13:54:26 -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; dkim=pass header.i=@linutronix.de header.s=2020 header.b="clg/w3v6"; dkim=neutral (no key) header.i=@linutronix.de; 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; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=linutronix.de Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229651AbjFRUuV (ORCPT + 99 others); Sun, 18 Jun 2023 16:50:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48664 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229579AbjFRUuA (ORCPT ); Sun, 18 Jun 2023 16:50:00 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0D37D123; Sun, 18 Jun 2023 13:49:59 -0700 (PDT) Date: Sun, 18 Jun 2023 20:49:57 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1687121397; 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=Ulay+IJn+o8JcX+jgajJWMne8J/Z4XZyhw+hfQwiS6Q=; b=clg/w3v6DdGuGOuAqMd2SshrVg55b3AC3DU7vwRmL7p65CdFOAxwNZwxoIEP4ymV2JDJ3x WmYM77MPI4z8EDvILnoBG77wdh5CWhFF/ZWMs7f1/D9XZclgkisNH8j8DjRXWV9MF0C2Df SRpztuAI09gzBQh3CfbaNKTBW38okMaL9OrlGMsQeUokKQY1GFEpPHJMHTtbQ6KQxV3Bg8 XGqjhgQldJqqE6kSdOnMkXjTsLwx0QZKTCAtVTmLJU4wQtyjOQpoWrKcawOxzO1JW8kaGy qDJUJ+gQ+C0CL4LAiKFFngoiz0Dk316PICSayggSYq0gJuSaxevtBG57Lb6jPA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1687121397; 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=Ulay+IJn+o8JcX+jgajJWMne8J/Z4XZyhw+hfQwiS6Q=; b=wGX5UD2uWBGM4fIXBt7GURPnUl2Iu2Ek/QYEVuF3fF9vj+CbWGhjYMLSOAWB/C5oRYKLDO wKDFzEQqri8Y4wBg== From: "tip-bot2 for Thomas Gleixner" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: timers/core] posix-timers: Clarify posix_timer_fn() comments Cc: Thomas Gleixner , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <874jnrdmrq.ffs@tglx> References: <874jnrdmrq.ffs@tglx> MIME-Version: 1.0 Message-ID: <168712139719.404.2275002250356402873.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,SPF_HELO_NONE, SPF_PASS,T_SCC_BODY_TEXT_LINE autolearn=ham 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: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1764176999689763952?= X-GMAIL-MSGID: =?utf-8?q?1769075288762729983?= The following commit has been merged into the timers/core branch of tip: Commit-ID: 84999b8bdb4969816c7bb7c14c3a55ed42aa4b94 Gitweb: https://git.kernel.org/tip/84999b8bdb4969816c7bb7c14c3a55ed42aa4b94 Author: Thomas Gleixner AuthorDate: Thu, 01 Jun 2023 21:07:37 +02:00 Committer: Thomas Gleixner CommitterDate: Sun, 18 Jun 2023 22:41:52 +02:00 posix-timers: Clarify posix_timer_fn() comments Make the issues vs. SIG_IGN understandable and remove the 15 years old promise that a proper solution is already on the horizon. Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/874jnrdmrq.ffs@tglx --- kernel/time/posix-timers.c | 62 +++++++++++++++++++------------------ 1 file changed, 32 insertions(+), 30 deletions(-) diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c index d8d2169..ae8799e 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -326,11 +326,11 @@ int posix_timer_event(struct k_itimer *timr, int si_private) } /* - * This function gets called when a POSIX.1b interval timer expires. It - * is used as a callback from the kernel internal timer. The - * run_timer_list code ALWAYS calls with interrupts on. - - * This code is for CLOCK_REALTIME* and CLOCK_MONOTONIC* timers. + * This function gets called when a POSIX.1b interval timer expires from + * the HRTIMER interrupt (soft interrupt on RT kernels). + * + * Handles CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_BOOTTIME and CLOCK_TAI + * based timers. */ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer) { @@ -348,9 +348,10 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer) if (posix_timer_event(timr, si_private)) { /* - * signal was not sent because of sig_ignor - * we will not get a call back to restart it AND - * it should be restarted. + * The signal was not queued due to SIG_IGN. As a + * consequence the timer is not going to be rearmed from + * the signal delivery path. But as a real signal handler + * can be installed later the timer must be rearmed here. */ if (timr->it_interval != 0) { ktime_t now = hrtimer_cb_get_time(timer); @@ -359,34 +360,35 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer) * FIXME: What we really want, is to stop this * timer completely and restart it in case the * SIG_IGN is removed. This is a non trivial - * change which involves sighand locking - * (sigh !), which we don't want to do late in - * the release cycle. + * change to the signal handling code. + * + * For now let timers with an interval less than a + * jiffie expire every jiffie and recheck for a + * valid signal handler. + * + * This avoids interrupt starvation in case of a + * very small interval, which would expire the + * timer immediately again. * - * For now we just let timers with an interval - * less than a jiffie expire every jiffie to - * avoid softirq starvation in case of SIG_IGN - * and a very small interval, which would put - * the timer right back on the softirq pending - * list. By moving now ahead of time we trick - * hrtimer_forward() to expire the timer - * later, while we still maintain the overrun - * accuracy, but have some inconsistency in - * the timer_gettime() case. This is at least - * better than a starved softirq. A more - * complex fix which solves also another related - * inconsistency is already in the pipeline. + * Moving now ahead of time by one jiffie tricks + * hrtimer_forward() to expire the timer later, + * while it still maintains the overrun accuracy + * for the price of a slight inconsistency in the + * timer_gettime() case. This is at least better + * than a timer storm. + * + * Only required when high resolution timers are + * enabled as the periodic tick based timers are + * automatically aligned to the next tick. */ -#ifdef CONFIG_HIGH_RES_TIMERS - { - ktime_t kj = NSEC_PER_SEC / HZ; + if (IS_ENABLED(CONFIG_HIGHRES_TIMERS)) { + ktime_t kj = TICK_NSEC; if (timr->it_interval < kj) now = ktime_add(now, kj); } -#endif - timr->it_overrun += hrtimer_forward(timer, now, - timr->it_interval); + + timr->it_overrun += hrtimer_forward(timer, now, timr->it_interval); ret = HRTIMER_RESTART; ++timr->it_requeue_pending; timr->it_active = 1;