[PATCH 2/7] cputime: Remove extra cost in task_cputime

From: Frederic Weisbecker
Date: Thu Nov 19 2015 - 10:51:13 EST

From: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx>

There is an extra cost in task_cputime() and task_cputime_scaled() when
nohz_full is not activated. When vtime accounting is not enabled, we
don't need to get deltas of utime and stime under vtime seqlock.

This patch removes that cost with adding a shortcut route if vtime
accounting is not enabled.

Use context_tracking_is_enabled() to check if vtime is accounting on
some cpu, in which case only we need to check the tickless cputime delta.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Hiroshi Shimamoto <h-shimamoto@xxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
kernel/sched/cputime.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 05de80b..1128d4b 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -853,6 +853,14 @@ void task_cputime(struct task_struct *t, cputime_t *utime, cputime_t *stime)
cputime_t udelta, sdelta;

+ if (!context_tracking_is_enabled()) {
+ if (utime)
+ *utime = t->utime;
+ if (stime)
+ *stime = t->stime;
+ return;
+ }
fetch_task_cputime(t, utime, stime, &t->utime,
&t->stime, &udelta, &sdelta);
if (utime)
@@ -866,6 +874,14 @@ void task_cputime_scaled(struct task_struct *t,
cputime_t udelta, sdelta;

+ if (!context_tracking_is_enabled()) {
+ if (utimescaled)
+ *utimescaled = t->utimescaled;
+ if (stimescaled)
+ *stimescaled = t->stimescaled;
+ return;
+ }
fetch_task_cputime(t, utimescaled, stimescaled,
&t->utimescaled, &t->stimescaled, &udelta, &sdelta);
if (utimescaled)

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