From: Morten Rasmussen morten.rasmussen@arm.com
At task wake-up load-tracking isn't updated until the task is enqueued. The task's own view of its utilization contribution may therefore not be aligned with its contribution to the cfs_rq load-tracking which may have been updated in the meantime. Basically, the task's own utilization hasn't yet accounted for the sleep decay, while the cfs_rq may have (partially). Estimating the cfs_rq utilization in case the task is migrated at wake-up as task_rq(p)->cfs.avg.util_avg - p->se.avg.util_avg is therefore incorrect as the two load-tracking signals aren't time synchronized (different last update).
To solve this problem, this patch introduces task_util_wake() which computes the decayed task utilization based on the last update of the previous cpu's last load-tracking update. It is done without having to take the rq lock, similar to how it is done in remove_entity_load_avg().
Signed-off-by: Morten Rasmussen morten.rasmussen@arm.com --- kernel/sched/fair.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 69 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 17dcd8e..0385723 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5414,6 +5414,75 @@ static bool need_filter_task(struct task_struct *p) return 0; }
+/* + * task_util_wake: Returns an updated estimate of the utilization contribution + * of a waking task. At wake-up the task blocked utilization contribution + * (cfs_rq->avg) may have decayed while the utilization tracking of the task + * (se->avg) hasn't yet. + * Note that this estimate isn't perfectly accurate as the 1ms boundaries used + * for updating util_avg in __update_load_avg() are not considered here. This + * results in an error of up to 1ms utilization decay/accumulation which leads + * to an absolute util_avg error margin of 1024*1024/LOAD_AVG_MAX ~= 22 + * (for LOAD_AVG_MAX = 47742). + */ +static inline int task_util_wake(struct task_struct *p) +{ + struct cfs_rq *prev_cfs_rq = &task_rq(p)->cfs; + struct sched_avg *psa = &p->se.avg; + u64 cfs_rq_last_update, p_last_update, delta; + u32 util_decayed; + + p_last_update = psa->last_update_time; + + /* + * Task on rq (exec()) should be load-tracking aligned already. + * New tasks have no history and should use the init value. + */ + if (p->se.on_rq || !p_last_update) + return task_util(p); + + cfs_rq_last_update = cfs_rq_last_update_time(prev_cfs_rq); + delta = cfs_rq_last_update - p_last_update; + + if ((s64)delta <= 0) + return task_util(p); + + delta >>= 20; + + if (!delta) + return task_util(p); + + util_decayed = decay_load((u64)psa->util_sum, delta); + util_decayed /= LOAD_AVG_MAX; + + /* + * psa->util_avg can be slightly out of date as it is only updated + * when a 1ms boundary is crossed. + * See 'decayed' in __update_load_avg() + */ + util_decayed = min_t(unsigned long, util_decayed, task_util(p)); + + return util_decayed; +} + +/* + * cpu_util_wake: Compute cpu utilization with any contributions from + * the waking task p removed. + */ +static int cpu_util_wake(int cpu, struct task_struct *p) +{ + unsigned long util, capacity; + + /* Task has no contribution or is new */ + if (cpu != task_cpu(p) || !p->se.avg.last_update_time) + return cpu_util(cpu); + + capacity = capacity_orig_of(cpu); + util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util_wake(p), 0); + + return (util >= capacity) ? capacity : util; +} + #ifdef CONFIG_SCHED_TUNE
static long -- 1.9.1