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authorFrederic Weisbecker <fweisbec@gmail.com>2012-11-28 17:00:57 +0100
committerFrederic Weisbecker <fweisbec@gmail.com>2012-11-28 17:08:20 +0100
commitfa09205783d11cc05122ad6e4ce06074624b2c0c (patch)
treeee13b0a24856952b06b8ad92640ef90602fd4a86 /kernel/sched/cputime.c
parentd37f761dbd276790f70dcf73a287fde2c3464482 (diff)
downloadlinaro-lsk-fa09205783d11cc05122ad6e4ce06074624b2c0c.tar.gz
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>
Diffstat (limited to 'kernel/sched/cputime.c')
-rw-r--r--kernel/sched/cputime.c18
1 files changed, 16 insertions, 2 deletions
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 220fdc4db77..b7f73176862 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);