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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>
This commit is contained in:
Frederic Weisbecker 2012-11-28 17:00:57 +01:00
parent d37f761dbd
commit fa09205783
1 changed files with 16 additions and 2 deletions
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18
kernel/sched/cputime.c
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@ -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);