Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752385AbbHRIip (ORCPT ); Tue, 18 Aug 2015 04:38:45 -0400 Received: from www.linutronix.de ([62.245.132.108]:56020 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751349AbbHRIil (ORCPT ); Tue, 18 Aug 2015 04:38:41 -0400 Date: Tue, 18 Aug 2015 10:38:15 +0200 (CEST) From: Thomas Gleixner To: John Stultz cc: lkml , Shaohua Li , Prarit Bhargava , Richard Cochran , Daniel Lezcano , Ingo Molnar Subject: Re: [PATCH 8/9] clocksource: Improve unstable clocksource detection In-Reply-To: Message-ID: References: <1439844063-7957-1-git-send-email-john.stultz@linaro.org> <1439844063-7957-9-git-send-email-john.stultz@linaro.org> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001,URIBL_BLOCKED=0.001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2864 Lines: 69 On Mon, 17 Aug 2015, John Stultz wrote: > On Mon, Aug 17, 2015 at 3:04 PM, Thomas Gleixner wrote: > > On Mon, 17 Aug 2015, John Stultz wrote: > > > >> From: Shaohua Li > >> > >> >From time to time we saw TSC is marked as unstable in our systems, while > > > > Stray '>' > > > >> the CPUs declare to have stable TSC. Looking at the clocksource unstable > >> detection, there are two problems: > >> - watchdog clock source wrap. HPET is the most common watchdog clock > >> source. It's 32-bit and runs in 14.3Mhz. That means the hpet counter > >> can wrap in about 5 minutes. > >> - threshold isn't scaled against interval. The threshold is 0.0625s in > >> 0.5s interval. What if the actual interval is bigger than 0.5s? > >> > >> The watchdog runs in a timer bh, so hard/soft irq can defer its running. > >> Heavy network stack softirq can hog a cpu. IPMI driver can disable > >> interrupt for a very long time. > > > > And they hold off the timer softirq for more than a second? Don't you > > think that's the problem which needs to be fixed? > > Though this is an issue I've experienced (and tried unsuccessfully to > fix in a more complicated way) with the RT kernel, where high priority > tasks blocked the watchdog long enough that we'd disqualify the TSC. Did it disqualify the watchdog due to HPET wraparounds (5 minutes) or due to the fixed threshold being applied? > > So 'fixing' the watchdog is the wrong approach. Fixing the stuff which > > prevents the watchdog to run is the proper thing to do. > > I'm not sure here. I feel like these delay-caused false positives > (I've seen similar reports w/ VMs being stalled) are more common then > one-off SMI TSC skews. Yes, they are more common, but the other issues are reality as well. > There are hard lines in the timekeeping code, where we do say: Don't > delay us past X or we can't really handle it, but in this case, the > main clocksource is fine and the limit is being caused by the > watchdog. So I think some sort of a solution to remove this > restriction would be good. We don't want to needlessly punish fine > hardware because our checks for bad hardware add extra restrictions. No argument here. Though fine hardware has an escape route already to avoid the watchdog business alltogether (tsc=reliable on the command line). > That said, I agree the "should"s and other vague qualifiers in the > commit description you point out should have more specifics to back > things up. And I'm fine delaying this (and the follow-on) patch until > those details are provided. Fair enough. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/