From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753946AbbCMJDa (ORCPT ); Fri, 13 Mar 2015 05:03:30 -0400 Received: from terminus.zytor.com ([198.137.202.10]:57779 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752558AbbCMJDZ (ORCPT ); Fri, 13 Mar 2015 05:03:25 -0400 Date: Fri, 13 Mar 2015 02:02:50 -0700 From: tip-bot for John Stultz Message-ID: Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, torvalds@linux-foundation.org, mingo@kernel.org, sboyd@codeaurora.org, john.stultz@linaro.org, richardcochran@gmail.com, davej@codemonkey.org.uk, prarit@redhat.com, peterz@infradead.org, hpa@zytor.com Reply-To: tglx@linutronix.de, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, john.stultz@linaro.org, mingo@kernel.org, sboyd@codeaurora.org, prarit@redhat.com, davej@codemonkey.org.uk, richardcochran@gmail.com, peterz@infradead.org, hpa@zytor.com In-Reply-To: <1426133800-29329-7-git-send-email-john.stultz@linaro.org> References: <1426133800-29329-7-git-send-email-john.stultz@linaro.org> To: linux-tip-commits@vger.kernel.org Subject: [tip:timers/core] timekeeping: Try to catch clocksource delta underflows Git-Commit-ID: 057b87e3161d1194a095718f9918c01b2c389e74 X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 057b87e3161d1194a095718f9918c01b2c389e74 Gitweb: http://git.kernel.org/tip/057b87e3161d1194a095718f9918c01b2c389e74 Author: John Stultz AuthorDate: Wed, 11 Mar 2015 21:16:34 -0700 Committer: Ingo Molnar CommitDate: Fri, 13 Mar 2015 08:07:05 +0100 timekeeping: Try to catch clocksource delta underflows In the case where there is a broken clocksource where there are multiple actual clocks that aren't perfectly aligned, we may see small "negative" deltas when we subtract 'now' from 'cycle_last'. The values are actually negative with respect to the clocksource mask value, not necessarily negative if cast to a s64, but we can check by checking the delta to see if it is a small (relative to the mask) negative value (again negative relative to the mask). If so, we assume we jumped backwards somehow and instead use zero for our delta. Signed-off-by: John Stultz Cc: Dave Jones Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Prarit Bhargava Cc: Richard Cochran Cc: Stephen Boyd Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1426133800-29329-7-git-send-email-john.stultz@linaro.org Signed-off-by: Ingo Molnar --- kernel/time/timekeeping.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 657414c..187149b 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -148,6 +148,13 @@ static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr) /* calculate the delta since the last update_wall_time */ delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); + /* + * Try to catch underflows by checking if we are seeing small + * mask-relative negative values. + */ + if (unlikely((~delta & tkr->mask) < (tkr->mask >> 3))) + delta = 0; + /* Cap delta value to the max_cycles values to avoid mult overflows */ if (unlikely(delta > tkr->clock->max_cycles)) delta = tkr->clock->max_cycles;