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>

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 220fdc4db770c0b97992de5416800f00bc813f47..b7f7317686251b8b4b026a0eb8288ac5a3e7ef3e 100644 (file)
--- 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);