On Tue, Jan 21, 2014 at 12:00 PM, Vincent Guittot vincent.guittot@linaro.org wrote:
Le 21 janv. 2014 19:39, bsegall@google.com a écrit :
Vincent Guittot vincent.guittot@linaro.org writes:
With the current implementation, the load average statistics of a sched entity change according to other activity on the CPU even if this activity is done between the running window of the sched entity and have no influence on the running duration of the task.
When a task wakes up on the same CPU, we currently update last_runnable_update with the return of __synchronize_entity_decay without updating the runnable_avg_sum and runnable_avg_period accordingly. In fact, we have to sync the load_contrib of the se with the rq's blocked_load_contrib before removing it from the latter (with __synchronize_entity_decay) but we must keep last_runnable_update unchanged for updating runnable_avg_sum/period during the next update_entity_load_avg.
... Gah, that's correct, we had this right the first time. Could you do this as a full revert of 282cf499f03ec1754b6c8c945c9674b02631fb0f (ie remove the now inaccurate comment, or maybe replace it with a correct one).
Ok i'm going to remove comment as well and replace it with a new description
I think I need to go through and do a comments patch like we did with the wake-affine math; it's too easy to make a finicky mistake like this when not touching this path for a while.
OK, so there are two numerical components we're juggling here:
1) The actual quotient for the current runnable average, stored as (runnable_avg_sum / runnable_avg period). Last updated at last_runnable_update. 2) The last time we computed the quotient in (1) and accumulated it in within cfs_rq->{runnable, blocked}_load_avg, this is stored in load_avg_contrib. We track the passage of off-rq time and migrations against this value using decay_count. [ All of the values above are stored on se / se->avg ]
When we are re-enqueuing something and we wish to remove its contribution from blocked_load_avg, we must update load_avg_contrib in (2) using the total time it spent off rq (using a jiffy rounded approximation in decay_count). However, Alex's patch (which this reverts) also adjusted the quotient by modifying its last update time so as to make it look up-to-date, effectively skipping the most recent idle span.
I think we could make the connection between (1) and (2) more explicit if we moved the subsequent "if (wakeup) logic" within the else. We can then have a comment that refers to (1) and (2) explicitly, perhaps something like:
Task re-woke on same cpu (or else migrate_task_rq_fair() would have made count negative). Perform an approximate decay on load_avg_contrib to match blocked_load_avg, and compute a precise runnable_avg_sum quotient update that will be accumulated into runnable_load_avg below.
Signed-off-by: Vincent Guittot vincent.guittot@linaro.org
kernel/sched/fair.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index e64b079..5b0ef90 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2370,8 +2370,7 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq, * would have made count negative); we must be careful to avoid * double-accounting blocked time after synchronizing decays. */
se->avg.last_runnable_update +=__synchronize_entity_decay(se)
<< 20;
__synchronize_entity_decay(se); } /* migrated tasks did not contribute to our blocked load */