Re: [patch] sched/cputime: Fix NO_HZ_FULL getrusage() monotonicity regression

[Peter Zijlstra <peterz@infradead.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 <umgwanakikbuti@gmail.com>
> 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: