Commit-ID: fa09205783d11cc05122ad6e4ce06074624b2c0c Gitweb: http://git.kernel.org/tip/fa09205783d11cc05122ad6e4ce06074624b2c0c Author: Frederic Weisbecker <fweisbec@xxxxxxxxx> AuthorDate: Wed, 28 Nov 2012 17:00:57 +0100 Committer: Frederic Weisbecker <fweisbec@xxxxxxxxx> CommitDate: Wed, 28 Nov 2012 17:08:20 +0100 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@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx> --- kernel/sched/cputime.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index 220fdc4..b7f7317 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); -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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