linux-kernel - Re: [PATCH 1/1] cputime: Make the reported utime+stime correspond to the actual runtime.

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Date:	Fri, 12 Jun 2015 13:01:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Fredrik Markstrom <fredrik.markstrom@...il.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/1] cputime: Make the reported utime+stime correspond to
 the actual runtime.

On Fri, Jun 12, 2015 at 12:16:57PM +0200, Peter Zijlstra wrote:
> On Fri, 2015-06-12 at 10:55 +0200, Fredrik Markstrom wrote:
> > The scaling mechanism might sometimes cause top to report >100%
> > (sometimes > 1000%) cpu usage for a single thread. This patch makes
> > sure that stime+utime corresponds to the actual runtime of the thread.
> 
> This Changelog is inadequate, it does not explain the actual problem.
> 
> > +static DEFINE_SPINLOCK(prev_time_lock);
> 
> global (spin)locks are bad.

Since you have a proglet handy to test this; does something like the
below help anything?

---
 kernel/sched/cputime.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index f5a64ffad176..3d3f60a555a0 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -613,6 +613,10 @@ static void cputime_adjust(struct task_cputime *curr,
 
 		stime = scale_stime((__force u64)stime,
 				    (__force u64)rtime, (__force u64)total);
+
+		if (stime < prev->stime)
+			stime = prev->stime;
+
 		utime = rtime - stime;
 	}
 
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