Messages in this thread | | | Subject | Re: [PATCH 1/7] sched: Fix pick_next_task() vs change pattern race | From | Valentin Schneider <> | Date | Fri, 8 Nov 2019 16:05:25 +0000 |
| |
On 08/11/2019 13:15, Peter Zijlstra wrote: > Fixes: 67692435c411 ("sched: Rework pick_next_task() slow-path") > Reported-by: Quentin Perret <qperret@google.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
I've been running the same reproducer as Quentin's for a similar length of time (~3h) and it's still going strong, so FWIW:
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
> --- > kernel/sched/core.c | 21 +++++++++++++++------ > kernel/sched/deadline.c | 40 ++++++++++++++++++++-------------------- > kernel/sched/fair.c | 15 ++++++++++++--- > kernel/sched/idle.c | 9 ++++++++- > kernel/sched/rt.c | 37 +++++++++++++++++++------------------ > kernel/sched/sched.h | 30 +++++++++++++++++++++++++++--- > kernel/sched/stop_task.c | 18 +++++++++++------- > 7 files changed, 112 insertions(+), 58 deletions(-) > > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3929,13 +3929,22 @@ pick_next_task(struct rq *rq, struct tas > } > > restart: > +#ifdef CONFIG_SMP
I suppose we could dump that in a core balance_prev_task() to avoid the inline #ifdeffery, but eh...
> /* > - * 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'. > + * We must do the balancing pass before put_next_task(), such > + * that when we release the rq->lock the task is in the same > + * state as before we took rq->lock. > + * > + * We can terminate the balance pass as soon as we know there is > + * a runnable task of @class priority or higher. > */ > - prev->sched_class->put_prev_task(rq, prev, rf); > - if (!rq->nr_running) > - newidle_balance(rq, rf); > + for_class_range(class, prev->sched_class, &idle_sched_class) { > + if (class->balance(rq, prev, rf)) > + break; > + } > +#endif > + > + put_prev_task(rq, prev); > > for_each_class(class) { > p = class->pick_next_task(rq, NULL, NULL);
> @@ -1469,6 +1469,22 @@ static void check_preempt_equal_prio(str > resched_curr(rq); > } > > +static int balance_rt(struct rq *rq, struct task_struct *p, struct rq_flags *rf) > +{ > + if (!on_rt_rq(&p->rt) && need_pull_rt_task(rq, p)) { > + /* > + * This is OK, because current is on_cpu, which avoids it being > + * picked for load-balance and preemption/IRQs are still > + * disabled avoiding further scheduler activity on it and we've > + * not yet started the picking loop. > + */ > + rq_unpin_lock(rq, rf); > + pull_rt_task(rq); > + rq_repin_lock(rq, rf); > + } > + > + return sched_stop_runnable(rq) || sched_dl_runnable(rq) || sched_rt_runnable(rq);
So we already have some dependencies on the class ordering (e.g. fair->idle), but I'm wondering if would it make sense to have these runnable functions be defined as sched_class callbacks?
e.g.
rt_sched_class.runnable = rt_runnable
w/ rt_runnable() just being a non-inlined sched_rt_runnable() you define further down the patch (or a wrapper to it). The balance return pattern could then become:
for_class_range(class, sched_class_highest, rt_sched_class->next) if (class->runnable(rq)) return true; return false;
(and replace rt_sched_class by whatever class' balance callback this is)
It's a bit neater, but I'm pretty sure it's going to run worse :/ The only unaffected one would be fair, since newidle_balance() already does that "for free".
> +} > #endif /* CONFIG_SMP */ > > /*
| |