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| author | Frederic Weisbecker <fweisbec@gmail.com> | 2012-11-28 17:00:57 +0100 |
|---|---|---|
| committer | Frederic Weisbecker <fweisbec@gmail.com> | 2012-11-28 17:08:20 +0100 |
| commit | fa09205783d11cc05122ad6e4ce06074624b2c0c (patch) | |
| tree | ee13b0a24856952b06b8ad92640ef90602fd4a86 /kernel/sched/cputime.c | |
| parent | cputime: Consolidate cputime adjustment code (diff) | |
| download | linux-fa09205783d11cc05122ad6e4ce06074624b2c0c.tar.xz linux-fa09205783d11cc05122ad6e4ce06074624b2c0c.zip | |
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.c | 18 |
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); |