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SubjectRe: [PATCH -next 1/1] sched/rt: Try to restart rt period timer when rt runtime exceeded
On Mon, Nov 15, 2021 at 01:46:28AM +0000, Li Hua wrote:
> When rt_runtime is modified from -1 to a valid control value, it may
> cause the task to be throttled all the time. Operations like the following
> will trigger the bug. E.g:
> 1. echo -1 > /proc/sys/kernel/sched_rt_runtime_us
> 2. Run a FIFO task named A that executes while(1)
> 3. echo 950000 > /proc/sys/kernel/sched_rt_runtime_us
>
> When rt_runtime is -1, The rt period timer will not be activated when task A
> enqueued. And then the task will be throttled after setting rt_runtime to
> 950,000. The task will always be throttled because the rt period timer is not
> activated.
>
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Li Hua <hucool.lihua@huawei.com>
> ---
> kernel/sched/rt.c | 26 +++++++++++++++++++++++++-
> 1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index bb945f8faeca..630f2cbe37d0 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -947,6 +947,23 @@ static inline int rt_se_prio(struct sched_rt_entity *rt_se)
> return rt_task_of(rt_se)->prio;
> }
>
> +static inline void try_start_rt_bandwidth(struct rt_bandwidth *rt_b)
> +{
> + raw_spin_lock(&rt_b->rt_runtime_lock);
> + if (!rt_bandwidth_enabled() || rt_b->rt_runtime == RUNTIME_INF) {
> + raw_spin_unlock(&rt_b->rt_runtime_lock);
> + return;
> + }
> +
> + if (!rt_b->rt_period_active) {
> + rt_b->rt_period_active = 1;
> + hrtimer_forward_now(&rt_b->rt_period_timer, rt_b->rt_period);
> + hrtimer_start_expires(&rt_b->rt_period_timer,
> + HRTIMER_MODE_ABS_PINNED_HARD);
> + }
> + raw_spin_unlock(&rt_b->rt_runtime_lock);
> +}

This is almost a verbatim copy of start_rt_bandwidth() surely we can do
better.

> +
> static int sched_rt_runtime_exceeded(struct rt_rq *rt_rq)
> {
> u64 runtime = sched_rt_runtime(rt_rq);
> @@ -1027,11 +1044,16 @@ static void update_curr_rt(struct rq *rq)
> struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
>
> if (sched_rt_runtime(rt_rq) != RUNTIME_INF) {
> + int exceeded;
> +
> raw_spin_lock(&rt_rq->rt_runtime_lock);
> rt_rq->rt_time += delta_exec;
> - if (sched_rt_runtime_exceeded(rt_rq))
> + exceeded = sched_rt_runtime_exceeded(rt_rq);
> + if (exceeded)
> resched_curr(rq);
> raw_spin_unlock(&rt_rq->rt_runtime_lock);
> + if (exceeded)
> + try_start_rt_bandwidth(sched_rt_bandwidth(rt_rq));
> }
> }
> }
> @@ -2905,8 +2927,10 @@ static int sched_rt_global_validate(void)
>
> static void sched_rt_do_global(void)
> {
> + raw_spin_lock(&def_rt_bandwidth.rt_runtime_lock);
> def_rt_bandwidth.rt_runtime = global_rt_runtime();
> def_rt_bandwidth.rt_period = ns_to_ktime(global_rt_period());
> + raw_spin_unlock(&def_rt_bandwidth.rt_runtime_lock);

And that's just wrong I think; did you test this with lockdep enabled?
IIRC this lock is irq-safe, it has to be if you're using it form a timer
context.


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