Re: [PATCH] sched/fair: sanitize vruntime of entity being placed

[Chen Yu]

Hi Roman, Qiao,
On 2023-01-27 at 17:32:30 +0100, Roman Kagan wrote:
> From: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
> 
> When a scheduling entity is placed onto cfs_rq, its vruntime is pulled
> to the base level (around cfs_rq->min_vruntime), so that the entity
> doesn't gain extra boost when placed backwards.
> 
> However, if the entity being placed wasn't executed for a long time, its
> vruntime may get too far behind (e.g. while cfs_rq was executing a
> low-weight hog), which can inverse the vruntime comparison due to s64
> overflow.  This results in the entity being placed with its original
> vruntime way forwards, so that it will effectively never get to the cpu.
>
Looks interesting,
case 1:
  se->vruntime = 1, cfs_rq->min_vruntime = ULONG_MAX
  ==> max = 1
case 2:
  se->vruntime = 1, cfs_rq->min_vruntime = LONG_MAX
  ==> max = LONG_MAX

May I know if the issue you described above is in case 1? We want
the max to be ULONG_MAX but it returns 1 because of s64
comparison? Then max = 1 is incorrectly used as se's vruntime?
Could you please elaborate a little more about this issue?
> To prevent that, ignore the vruntime of the entity being placed if it
> didn't execute for much longer than the characteristic sheduler time
> scale.
> 
> [rkagan: formatted, adjusted commit log, comments, cutoff value]
> Co-developed-by: Roman Kagan <rkagan@xxxxxxxxx>
> Signed-off-by: Roman Kagan <rkagan@xxxxxxxxx>
> ---
> @zhangqiao22, I took the liberty to put you as the author of the patch,
> as this is essentially what you posted for discussion, with minor
> tweaks.  Please stamp with your s-o-b if you're ok with it.
> 
>  kernel/sched/fair.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0f8736991427..d6cf131ebb0b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4656,6 +4656,7 @@ static void
>  place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>  {
>  	u64 vruntime = cfs_rq->min_vruntime;
> +	u64 sleep_time;
>  
>  	/*
>  	 * The 'current' period is already promised to the current tasks,
> @@ -4685,8 +4686,18 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>  		vruntime -= thresh;
>  	}
>  
> -	/* ensure we never gain time by being placed backwards. */
> -	se->vruntime = max_vruntime(se->vruntime, vruntime);
> +	/*
> +	 * Pull vruntime of the entity being placed to the base level of
> +	 * cfs_rq, to prevent boosting it if placed backwards.  If the entity
> +	 * slept for a long time, don't even try to compare its vruntime with
> +	 * the base as it may be too far off and the comparison may get
> +	 * inversed due to s64 overflow.
> +	 */
> +	sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
If I understand correctly, se->exec_start is just updated by enqueue_entity()->update_curr(cfs_rq),
then place_entity() in invoked here, I'm not sure if sleep_time above
could reflect the real sleep time. Maybe something like:
rq_clock_task(rq_of(cfs_rq)) - se->time_stamp_dequeued ?

thanks,
Chenyu