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Commit fa092057 authored by Frederic Weisbecker's avatar Frederic Weisbecker
Browse files

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: default avatarFrederic 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>
parent d37f761d

kernel/sched/cputime.c

+16 −2
Original line number Diff line number Diff line
@@ -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);