cputime: Comment cputime's adjusting code

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 <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 220fdc4db770..b7f731768625 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);