Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Oct 23, 2020 at 01:25:38PM +0800, Li, Aubrey wrote:
> >>> @@ -2517,6 +2528,7 @@ const struct sched_class dl_sched_class
> >>>
> >>>  #ifdef CONFIG_SMP
> >>>       .balance                = balance_dl,
> >>> +     .pick_task              = pick_task_dl,
> >>>       .select_task_rq         = select_task_rq_dl,
> >>>       .migrate_task_rq        = migrate_task_rq_dl,
> >>>       .set_cpus_allowed       = set_cpus_allowed_dl,
> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>> index dbd9368a959d..bd6aed63f5e3 100644
> >>> --- a/kernel/sched/fair.c
> >>> +++ b/kernel/sched/fair.c
> >>> @@ -4450,7 +4450,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> >>>        * Avoid running the skip buddy, if running something else can
> >>>        * be done without getting too unfair.
> >>>        */
> >>> -     if (cfs_rq->skip == se) {
> >>> +     if (cfs_rq->skip && cfs_rq->skip == se) {
> >>>               struct sched_entity *second;
> >>>
> >>>               if (se == curr) {
> >>> @@ -6976,6 +6976,35 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
> >>>               set_last_buddy(se);
> >>>  }
> >>>
> >>> +#ifdef CONFIG_SMP
> >>> +static struct task_struct *pick_task_fair(struct rq *rq)
> >>> +{
> >>> +     struct cfs_rq *cfs_rq = &rq->cfs;
> >>> +     struct sched_entity *se;
> >>> +
> >>> +     if (!cfs_rq->nr_running)
> >>> +             return NULL;
> >>> +
> >>> +     do {
> >>> +             struct sched_entity *curr = cfs_rq->curr;
> >>> +
> >>> +             se = pick_next_entity(cfs_rq, NULL);
> >>> +
> >>> +             if (curr) {
> >>> +                     if (se && curr->on_rq)
> >>> +                             update_curr(cfs_rq);
> >>> +
> >>> +                     if (!se || entity_before(curr, se))
> >>> +                             se = curr;
> >>> +             }
> >>> +
> >>> +             cfs_rq = group_cfs_rq(se);
> >>> +     } while (cfs_rq);
> >>> ++
> >>> +     return task_of(se);
> >>> +}
> >>> +#endif
> >>
> >> One of my machines hangs when I run uperf with only one message:
> >> [  719.034962] BUG: kernel NULL pointer dereference, address: 0000000000000050
> >>
> >> Then I replicated the problem on my another machine(no serial console),
> >> here is the stack by manual copy.
> >>
> >> Call Trace:
> >>  pick_next_entity+0xb0/0x160
> >>  pick_task_fair+0x4b/0x90
> >>  __schedule+0x59b/0x12f0
> >>  schedule_idle+0x1e/0x40
> >>  do_idle+0x193/0x2d0
> >>  cpu_startup_entry+0x19/0x20
> >>  start_secondary+0x110/0x150
> >>  secondary_startup_64_no_verify+0xa6/0xab
> > 
> > Interesting. Wondering if we screwed something up in the rebase.
> > 
> > Questions:
> > 1. Does the issue happen if you just apply only up until this patch,
> > or the entire series?
> 
> I applied the entire series and just find a related patch to report the
> issue.

Ok.

> > 2. Do you see the issue in v7? Not much if at all has changed in this
> > part of the code from v7 -> v8 but could be something in the newer
> > kernel.
> > 
> 
> IIRC, I can run uperf successfully on v7.
> I'm on tip/master 2d3e8c9424c9 (origin/master) "Merge branch 'linus'."
> Please let me know if this is a problem, or you have a repo I can pull
> for testing.

Here is a repo with v8 series on top of v5.9 release:
https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/log/?h=coresched-v5.9

thanks,

 - Joel