Re: [PATCH] sched/eevdf: Avoid NULL in pick_eevdf

[Xuewen Yan]

Hi Abel

On Mon, Nov 20, 2023 at 4:49 PM Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
>
> Hi Xuewen, the pick part has been re-worked, would you please re-test
> with the newest branch?
>
> Thanks,
>         Abel
>
> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/?h=sched%2Fcore

These patches would be merged into 6.6? If not, the pr_err also has a deadlock.
And should be changed to printk_deferred.


954 static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
  955 {
  956         struct sched_entity *se = __pick_eevdf(cfs_rq);
  957
  958         if (!se) {
  959                 struct sched_entity *left = __pick_first_entity(cfs_rq);
  960                 if (left) {
  961                         pr_err("EEVDF scheduling fail, picking
leftmost\n"); <<<<
  962                         return left;
  963                 }
  964         }
  965
  966         return se;
  967 }


>
> On 11/20/23 3:38 PM, Xuewen Yan Wrote:
> > Now in pick_eevdf function, add the pick_first_entity to prevent
> > picking null when using eevdf, however, the leftmost may be null.
> > As a result, it would cause oops because the se is NULL.
> >
> > Fix this by compare the curr and left, if the left is null, set
> > the se be curr.
> >
> > Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
> > Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 18 +++++++++++++++++-
> >   1 file changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index d7a3c63a2171..10916f6778ac 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -951,12 +951,28 @@ static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq)
> >       return NULL;
> >   }
> >
> > +/* Just simply choose the se with the smallest vruntime */
> > +static struct sched_entity *__pick_cfs(struct cfs_rq *cfs_rq)
> > +{
> > +     struct sched_entity *curr = cfs_rq->curr;
> > +     struct sched_entity *left = __pick_first_entity(cfs_rq);
> > +
> > +     /*
> > +      * If curr is set we have to see if its left of the leftmost entity
> > +      * still in the tree, provided there was anything in the tree at all.
> > +      */
> > +     if (!left || (curr && entity_before(curr, left)))
> > +             left = curr;
> > +
> > +     return left;
> > +}
> > +
> >   static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
> >   {
> >       struct sched_entity *se = __pick_eevdf(cfs_rq);
> >
> >       if (!se) {
> > -             struct sched_entity *left = __pick_first_entity(cfs_rq);
> > +             struct sched_entity *left = __pick_cfs(cfs_rq);
> >               if (left) {
> >                       pr_err("EEVDF scheduling fail, picking leftmost\n");
> >                       return left;