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SubjectRe: problem with "sched: revert back to per-rq vruntime"?
Mike Galbraith wrote:
> On Tue, 2008-12-30 at 13:37 -0600, Jayson King wrote:
>
>> Here's some times using bash, for example. Let me know if there are some
>> other tests I can do.
>>
>>
>> [jayson@atlas tmp]$ time bash -i -c 'exit'
>> exit
>>
>> real 0m0.753s
>> user 0m0.011s
>> sys 0m0.011s
>>
>
> Yeah, it's b0rked. The problem is that we call sched_slice() before the
> new task's load weight has been added to the runqueue.
>
> Normally this doesn't cause horrible problems, but when there is a +nice
> hog on the runqueue, the lighter the hog, and/or heavier the victim...
>
> [ 160.489402] bash:5858 vslice: 3840000000 weight: 1024 rq_weight: 15 inv_weight: 268435456
>
> ...the closer the victim's vruntime gets to the middle of next week.
>
> Not-signed-off diag hack.
>
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 5ad4440..3254213 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -449,13 +449,34 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> }
>
> /*
> + * A freshly forked task's load weight has not yet been added to the runqueue,
> + * so we can't call calc_delta_weight() and get sane results. Therefore, we roll
> + * our own so place_entity() can place this task in the right spot. The runqueue
> + * weight and nr_running will be updated when the task is enqueued.
> + */
> +static u64 sched_initial_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> + u64 slice = __sched_period(cfs_rq->nr_running + 1);
> +
> + for_each_sched_entity(se) {
> + struct load_weight lw = cfs_rq->load;
> +
> + lw.weight += se->load.weight;
> + lw.inv_weight = 0;
> + slice = calc_delta_mine(slice, se->load.weight, &lw);
> + }
> +
> + return slice;
> +}
> +
> +/*
> * We calculate the vruntime slice of a to be inserted task
> *
> * vs = s/w
> */
> static u64 sched_vslice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> {
> - return calc_delta_fair(sched_slice(cfs_rq, se), se);
> + return calc_delta_fair(sched_initial_slice(cfs_rq, se), se);
> }
>
> /*
>

I tested it. Now the times are back to normal.

# 2.6.28 under 100% nice CPU and Mike Galbraith patch
[jayson@atlas tmp]$ time bash -i -c 'exit'
exit

real 0m0.042s
user 0m0.006s
sys 0m0.003s



Jayson



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