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sched/eevdf: Fix avg_vruntime()

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The expectation is that placing a task at avg_vruntime() makes it
eligible. Turns out there is a corner case where this is not the case.

Specifically, avg_vruntime() relies on the fact that integer division
is a flooring function (eg. it discards the remainder). By this
property the value returned is slightly left of the true average.

However! when the average is a negative (relative to min_vruntime) the
effect is flipped and it becomes a ceil, with the result that the
returned value is just right of the average and thus not eligible.

Fixes: af4cf40470 ("sched/fair: Add cfs_rq::avg_vruntime")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
This commit is contained in:
Peter Zijlstra 2023-09-26 14:29:50 +02:00
parent 2f2fc17bab
commit 650cad561c
1 changed files with 9 additions and 1 deletions
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10
kernel/sched/fair.c
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@ -664,6 +664,10 @@ void avg_vruntime_update(struct cfs_rq *cfs_rq, s64 delta)
cfs_rq->avg_vruntime -= cfs_rq->avg_load * delta;
}
/*
* Specifically: avg_runtime() + 0 must result in entity_eligible() := true
* For this to be so, the result of this function must have a left bias.
*/
u64 avg_vruntime(struct cfs_rq *cfs_rq)
{
struct sched_entity *curr = cfs_rq->curr;
@ -677,8 +681,12 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
load += weight;
}
if (load)
if (load) {
/* sign flips effective floor / ceil */
if (avg < 0)
avg -= (load - 1);
avg = div_s64(avg, load);
}
return cfs_rq->min_vruntime + avg;
}