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