@@ -2916,12 +2916,12 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
/*
* Step 2: update *_avg.
*/
- sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX);
+ sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
if (cfs_rq) {
cfs_rq->runnable_load_avg =
- div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
+ div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
}
- sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
+ sa->util_avg = sa->util_sum / (LOAD_AVG_MAX - 1024 + sa->period_contrib);
return 1;
}
In the current implementation of load/util_avg, we assume that the ongoing time segment has fully elapsed, and util/load_sum is divided by LOAD_AVG_MAX, even if part of the time segment still remains to run. As a consequence, this remaining part is considered as idle time and generates unexpected variations of util_avg of a busy CPU in the range [1002..1024[ whereas util_avg should stay at 1023. In order to keep the metric stable, we should not consider the ongoing time segment when computing load/util_avg but only the segments that have already fully elapsed. But to not consider the current time segment adds unwanted latency in the load/util_avg responsivness especially when the time is scaled instead of the contribution. Instead of waiting for the current time segment to have fully elapsed before accounting it in load/util_avg, we can already account the elapsed part but change the range used to compute load/util_avg accordingly. At the very beginning of a new time segment, the past segments have been decayed and the max value is LOAD_AVG_MAX*y. At the very end of the current time segment, the max value becomes 1024(us) + LOAD_AVG_MAX*y which is equal to LOAD_AVG_MAX. In fact, the max value is sa->period_contrib + LOAD_AVG_MAX*y at any time in the time segment. Taking advantage of the fact that LOAD_AVG_MAX*y == LOAD_AVG_MAX-1024, the range becomes [0..LOAD_AVG_MAX-1024+sa->period_contrib]. As the elapsed part is already accounted in load/util_sum, we update the max value according to the current position in the time segment instead of removing its contribution. Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> --- Changes: -Correct typo in commit message: s/MAX_LOAD_AVG/LOAD_AVG_MAX/ and square bracket kernel/sched/fair.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) -- 2.7.4