From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756556AbdKDJO5 (ORCPT ); Sat, 4 Nov 2017 05:14:57 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:47672 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756478AbdKDJOz (ORCPT ); Sat, 4 Nov 2017 05:14:55 -0400 X-Google-Smtp-Source: ABhQp+QtAPHwbJWvONteldW5Hue77TyQUl3a+QZ/EzyM2eVspYkiUmIGtlAHb6wyzXNfhDUZr81zkw== From: Nicolai Stange To: Luiz Capitulino Cc: fweisbec@gmail.com, tglx@linutronix.de, mtosatti@redhat.com, williams@redhat.com, nicstange@gmail.com, linux-kernel@vger.kernel.org, John Stultz Subject: Re: [nohz_full/apic] multiple timer interrupts a second References: <20171103170703.46d72a31@redhat.com> Date: Sat, 04 Nov 2017 10:14:52 +0100 In-Reply-To: <20171103170703.46d72a31@redhat.com> (Luiz Capitulino's message of "Fri, 3 Nov 2017 17:12:03 -0400") Message-ID: <878tfm76cz.fsf@gmail.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Luiz, [John Stultz added to CC] On Fri, Nov 03 2017, Luiz Capitulino wrote: > [CC'ing lkml this time] > > I've observed that smp_apic_timer_interrupt() is sometimes called > two or more times a second on a nohz_full core which has a single > task taking 100% of the core. In one of the calls, hrtimer_interrupt() > runs tick_sched_timer(), but in others it doesn't call any handler. > Here's an example (Linus HEAD f34157878): > > <...>-1831 [008] 1060.115578: funcgraph_entry: | smp_apic_timer_interrupt() { > <...>-1831 [008] 1060.115578: funcgraph_entry: | hrtimer_interrupt() { > <...>-1831 [008] 1060.115578: hrtimer_expire_entry: hrtimer=0xffff8edfefd12d60 function=tick_sched_timer now=1060079001980 > <...>-1831 [008] 1060.115578: funcgraph_entry: 1.172 us | tick_sched_timer(); > <...>-1831 [008] 1060.115580: funcgraph_exit: 1.757 us | } > <...>-1831 [008] 1060.115580: hrtimer_start: hrtimer=0xffff8edfefd12d60 function=tick_sched_timer expires=1061079000000 softexpires=1061079000000 > <...>-1831 [008] 1060.115581: funcgraph_exit: 3.026 us | } > <...>-1831 [008] 1061.115577: funcgraph_entry: | smp_apic_timer_interrupt() { > <...>-1831 [008] 1061.115577: funcgraph_entry: 0.261 us | hrtimer_interrupt(); <---------- NO handler called > <...>-1831 [008] 1061.115578: funcgraph_exit: 1.349 us | } > <...>-1831 [008] 1061.115579: funcgraph_entry: | smp_apic_timer_interrupt() { > <...>-1831 [008] 1061.115579: funcgraph_entry: | hrtimer_interrupt() { > <...>-1831 [008] 1061.115579: hrtimer_expire_entry: hrtimer=0xffff8edfefd12d60 function=tick_sched_timer now=1061079001473 > <...>-1831 [008] 1061.115580: funcgraph_entry: 1.413 us | tick_sched_timer(); > <...>-1831 [008] 1061.115581: funcgraph_exit: 2.124 us | } > <...>-1831 [008] 1061.115582: hrtimer_start: hrtimer=0xffff8edfefd12d60 function=tick_sched_timer expires=1062079000000 softexpires=1062079000000 > <...>-1831 [008] 1061.115582: funcgraph_exit: 3.255 us | } > > Is this expected for some reason? > > I guess what's happening is that the deadline timer is firing > earlier than expected. From a few dozen to a few hundreds > nanoseconds earlier. When this happens, hrtimer_interrupt() > skips calling the hrtimer handler (since it's early) and the > apic is programmed to fire in the next microsecond. Exactly. > On further research I saw that Nicolai tried to fix a very similar > problem last year: > > commit 1a9e4c564ab174e53ed86def922804a5ddc63e7d > Author: Nicolai Stange > Date: Thu Jul 14 17:22:54 2016 +0200 > > x86/timers/apic: Fix imprecise timer interrupts by eliminating TSC clockevents frequency roundoff error > > I noticed the following bug/misbehavior on certain Intel systems: with a > single task running on a NOHZ CPU on an Intel Haswell, I recognized > that I did not only get the one expected local_timer APIC interrupt, but > two per second at minimum. (!) Note that there's also 6731b0d611a1 ("x86/timers/apic: Inform TSC deadline clockevent device about recalibration"). > Maybe this issue is still present? Yes it is. The reason is that NTP frequency adjustments would make it into the mono clocksource's frequency but not into the clock_event_device's. I sent a series back then [1] addressing this. However, in the meanwhile, I changed jobs and thus, got somehow distracted. My bad :/ The result is that this got merged only up to what corresponds to [10/28] in [1]. I'll pick this up again now and try to get the rest accepted for inclusion. Thanks, Nicolai [1] last time I sent this series as a whole can be found at http://lkml.kernel.org/r/20161119160055.12491-2-nicstange@gmail.com