Re: [PATCH] sched/fair: Always update_curr() before placing at enqueue

[Daniel Jordan]

On Fri, Oct 06, 2023 at 09:58:10PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 06, 2023 at 12:48:26PM -0400, Daniel Jordan wrote:
> > Placing wants current's vruntime and the cfs_rq's min_vruntime up to
> > date so that avg_runtime() is too, and similarly it wants the entity to
> > be re-weighted and lag adjusted so vslice and vlag are fresh, so always
> > do update_curr() and update_cfs_group() beforehand.
> > 
> > There doesn't seem to be a reason to treat the 'curr' case specially
> > after e8f331bcc270 since vruntime doesn't get normalized anymore.
> > 
> > Fixes: e8f331bcc270 ("sched/smp: Use lag to simplify cross-runqueue placement")
> > Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> > ---
> > 
> > Not sure what the XXX above place_entity() is for, maybe it can go away?
> > 
> > Based on tip/sched/core.
> > 
> >  kernel/sched/fair.c | 14 ++------------
> >  1 file changed, 2 insertions(+), 12 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 04fbcbda97d5f..db2ca9bf9cc49 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5047,15 +5047,6 @@ static inline bool cfs_bandwidth_used(void);
> >  static void
> >  enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >  {
> > -	bool curr = cfs_rq->curr == se;
> > -
> > -	/*
> > -	 * If we're the current task, we must renormalise before calling
> > -	 * update_curr().
> > -	 */
> > -	if (curr)
> > -		place_entity(cfs_rq, se, flags);
> > -
> >  	update_curr(cfs_rq);
> 
> IIRC part of the reason for this order is the:
> 
>   dequeue
>   update
>   enqueue
> 
> pattern we have all over the place. You don't want the enqueue to move
> time forward in this case.
> 
> Could be that all magically works, but please double check.

Yes, I wasn't thinking of the dequeue/update/enqueue places.
Considering these, it seems like there's more to fix (from before EEVDF
even).

Sorry for the delayed response, been staring for a while thinking I'd
have it all by the next day.  It'll take a bit longer to sort out all
the cases, but I'll keep going.