The reason for the scaling and monotonicity correction performed
by cputime_adjust() may not be immediately clear to the reviewer.
Add some comments to explain what happens there.
Signed-off-by: Frederic Weisbecker <fweis...@gmail.com>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Paul Gortmaker <paul.gortma...@windriver.com>
---
kernel/sched/cputime.c | 18 ++++++++++++++++--
1 files changed, 16 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 220fdc4..b7f7317 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -516,6 +516,10 @@ static cputime_t scale_utime(cputime_t utime, cputime_t
rtime, cputime_t total)
return (__force cputime_t) temp;
}
+/*
+ * Adjust tick based cputime random precision against scheduler
+ * runtime accounting.
+ */
static void cputime_adjust(struct task_cputime *curr,
struct cputime *prev,
cputime_t *ut, cputime_t *st)
@@ -524,8 +528,16 @@ static void cputime_adjust(struct task_cputime *curr,
utime = curr->utime;
total = utime + curr->stime;
+
/*
- * Use CFS's precise accounting:
+ * Tick based cputime accounting depend on random scheduling
+ * timeslices of a task to be interrupted or not by the timer.
+ * Depending on these circumstances, the number of these interrupts
+ * may be over or under-optimistic, matching the real user and system
+ * cputime with a variable precision.
+ *
+ * Fix this by scaling these tick based values against the total
+ * runtime accounted by the CFS scheduler.
*/
rtime = nsecs_to_cputime(curr->sum_exec_runtime);
@@ -535,7 +547,9 @@ static void cputime_adjust(struct task_cputime *curr,
utime = rtime;
/*
- * Compare with previous values, to keep monotonicity:
+ * If the tick based count grows faster than the scheduler one,
+ * the result of the scaling may go backward.
+ * Let's enforce monotonicity.
*/
prev->utime = max(prev->utime, utime);
prev->stime = max(prev->stime, rtime - prev->utime);
--
1.7.5.4
--
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