From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752707AbcHOIvc (ORCPT ); Mon, 15 Aug 2016 04:51:32 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:42217 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751488AbcHOIva (ORCPT ); Mon, 15 Aug 2016 04:51:30 -0400 Date: Mon, 15 Aug 2016 10:51:28 +0200 From: Peter Zijlstra To: Mike Galbraith Cc: LKML Subject: Re: [patch] sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression Message-ID: <20160815085128.GA30192@twins.programming.kicks-ass.net> References: <1470827669.15624.16.camel@gmail.com> <20160810123033.GM30192@twins.programming.kicks-ass.net> <1470855448.4840.2.camel@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1470855448.4840.2.camel@gmail.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 10, 2016 at 08:57:28PM +0200, Mike Galbraith wrote: > sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression > > Roughly 10% of the time, ltp testcase getrusage04 fails: > getrusage04 0 TINFO : Expected timers granularity is 4000 us > getrusage04 0 TINFO : Using 1 as multiply factor for max [us]time increment (1000+4000us)! > getrusage04 0 TINFO : utime: 0us; stime: 179us > getrusage04 0 TINFO : utime: 3751us; stime: 0us > getrusage04 1 TFAIL : getrusage04.c:133: stime increased > 5000us: > > If ->sum_exec_runtime has moved beyond the rtime of ->prev_cputime, but > no time has as yet been accounted to the task, bail. > > Fixes: 9d7fb0427648 ("sched/cputime: Guarantee stime + utime == rtime") > Signed-off-by: Mike Galbraith > Cc: stable@vger.kernel.org # 4.3+ > --- > kernel/sched/cputime.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > --- a/kernel/sched/cputime.c > +++ b/kernel/sched/cputime.c > @@ -606,6 +606,13 @@ static void cputime_adjust(struct task_c > stime = curr->stime; > utime = curr->utime; > > + /* > + * sum_exec_runtime has moved, but nothing has yet been > + * accounted to the task, there's nothing to update. > + */ > + if (utime + stime == 0) > + goto out; urgh... Valid scenario.. not sure about the solution though. This would mean the task has _no_ running time if it forever dodges the tick, which would be bad. Does something like so cure things too? --- kernel/sched/cputime.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index 9858266fb0b3..2ee83b200504 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -614,19 +614,25 @@ static void cputime_adjust(struct task_cputime *curr, stime = curr->stime; utime = curr->utime; - if (utime == 0) { - stime = rtime; + /* + * If either stime or both stime and utime are 0, assume all runtime is + * userspace. Once a task gets some ticks, the monotonicy code at + * 'update' will ensure things converge to the observed ratio. + */ + if (stime == 0) { + utime = rtime; goto update; } - if (stime == 0) { - utime = rtime; + if (utime == 0) { + stime = rtime; goto update; } stime = scale_stime((__force u64)stime, (__force u64)rtime, (__force u64)(stime + utime)); +update: /* * Make sure stime doesn't go backwards; this preserves monotonicity * for utime because rtime is monotonic. @@ -649,7 +655,6 @@ static void cputime_adjust(struct task_cputime *curr, stime = rtime - utime; } -update: prev->stime = stime; prev->utime = utime; out: