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SubjectRe: [PATCH 1/7] sched: Fix pick_next_task() vs change pattern race
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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 */
>
> /*

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