Messages in this thread | | | Date | Fri, 8 Nov 2019 11:02:12 +0000 | From | Quentin Perret <> | Subject | Re: NULL pointer dereference in pick_next_task_fair |
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On Thursday 07 Nov 2019 at 20:29:07 (+0100), Peter Zijlstra wrote: > I still havne't had food, but this here compiles...
And it seems to work, too :)
> @@ -3929,13 +3929,17 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > } > > restart: > - /* > - * Ensure that we put DL/RT tasks before the pick loop, such that they > - * can PULL higher prio tasks when we lower the RQ 'priority'. > - */ > - prev->sched_class->put_prev_task(rq, prev, rf); > - if (!rq->nr_running) > - newidle_balance(rq, rf); > +#ifdef CONFIG_SMP > + for (class = prev->sched_class; > + class != &idle_sched_class; > + class = class->next) { > + > + if (class->balance(rq, prev, rf)) > + break; > + } > +#endif > + > + put_prev_task(rq, prev);
Right, that looks much cleaner IMO. I'm thinking if we killed the special case for CFS above we could do with a single loop to iterate the classes, and you could fold ->balance() in ->pick_next_task() ... That would remove one call site to newidle_balance() too, which I think is good. Hackbench probably won't like that, though.
> for_each_class(class) { > p = class->pick_next_task(rq, NULL, NULL); > @@ -6201,7 +6205,7 @@ static struct task_struct *__pick_migrate_task(struct rq *rq) > for_each_class(class) { > next = class->pick_next_task(rq, NULL, NULL); > if (next) { > - next->sched_class->put_prev_task(rq, next, NULL); > + next->sched_class->put_prev_task(rq, next); > return next; > } > } > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index 2dc48720f189..b6c3fb10cf57 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -1778,15 +1778,9 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > return p; > } > > -static void put_prev_task_dl(struct rq *rq, struct task_struct *p, struct rq_flags *rf) > +static int balance_dl(struct rq *rq, struct task_struct *p, struct rq_flags *rf) > { > - update_curr_dl(rq); > - > - update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1); > - if (on_dl_rq(&p->dl) && p->nr_cpus_allowed > 1) > - enqueue_pushable_dl_task(rq, p); > -
Ah, and this can actually be done after the pull because the two are in fact mutually exclusive. And same thing for RT. Good :)
> - if (rf && !on_dl_rq(&p->dl) && need_pull_dl_task(rq, p)) { > + if (!on_dl_rq(&p->dl) && need_pull_dl_task(rq, p)) { > /* > * This is OK, because current is on_cpu, which avoids it being > * picked for load-balance and preemption/IRQs are still > @@ -1797,6 +1791,18 @@ static void put_prev_task_dl(struct rq *rq, struct task_struct *p, struct rq_fla > pull_dl_task(rq); > rq_repin_lock(rq, rf); > } > + > + return rq->dl.dl_nr_running > 0; > +} > + > + > +static void put_prev_task_dl(struct rq *rq, struct task_struct *p) > +{ > + update_curr_dl(rq); > + > + update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1); > + if (on_dl_rq(&p->dl) && p->nr_cpus_allowed > 1) > + enqueue_pushable_dl_task(rq, p); > } > > /* > @@ -2438,6 +2444,7 @@ const struct sched_class dl_sched_class = { > .check_preempt_curr = check_preempt_curr_dl, > > .pick_next_task = pick_next_task_dl, > + .balance = balance_dl, > .put_prev_task = put_prev_task_dl, > .set_next_task = set_next_task_dl, > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index a14487462b6c..6b983214e00f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6746,10 +6746,18 @@ done: __maybe_unused; > return NULL; > } > > +static int balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > +{ > + if (rq->cfs.nr_running) > + return 1; > + > + return newidle_balance(rq, rf) != 0;
And you can ignore the RETRY_TASK case here under the assumption that we must have tried to pull from RT/DL before ending up here ?
Thanks, Quentin
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