Re: [PATCH V4 1/3] sched/deadline: Replenishment timer should fire in the next period

From: Wanpeng Li
Date: Tue Mar 07 2017 - 03:02:31 EST


2017-03-02 22:10 GMT+08:00 Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>:
> Currently, the replenishment timer is set to fire at the deadline
> of a task. Although that works for implicit deadline tasks because the
> deadline is equals to the begin of the next period, that is not correct
> for constrained deadline tasks (deadline < period).
>
> For instance:
>
> f.c:
> --------------- %< ---------------
> int main (void)
> {
> for(;;);
> }
> --------------- >% ---------------
>
> # gcc -o f f.c
>
> # trace-cmd record -e sched:sched_switch \
> -e syscalls:sys_exit_sched_setattr \
> chrt -d --sched-runtime 490000000 \
> --sched-deadline 500000000 \
> --sched-period 1000000000 0 ./f
>
> # trace-cmd report | grep "{pid of ./f}"
>
> After setting parameters, the task is replenished and continue running
> until being throttled.
> f-11295 [003] 13322.113776: sys_exit_sched_setattr: 0x0
>
> The task is throttled after running 492318 ms, as expected.

This should be us.

Otherwise,

Reviewed-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

> f-11295 [003] 13322.606094: sched_switch: f:11295 [-1] R ==> \
> watchdog/3:32 [0]
>
> But then, the task is replenished 500719 ms after the first
> replenishment:
> <idle>-0 [003] 13322.614495: sched_switch: swapper/3:0 [120] R \
> ==> f:11295 [-1]
>
> Running for 490277 ms:
> f-11295 [003] 13323.104772: sched_switch: f:11295 [-1] R ==> \
> swapper/3:0 [120]
>
> Hence, in the first period, the task runs 2 * runtime, and that is a bug.
>
> During the first replenishment, the next deadline is set one period away.
> So the runtime / period starts to be respected. However, as the second
> replenishment took place in the wrong instant, the next replenishment
> will also be held in a wrong instant of time. Rather than occurring in
> the nth period away from the first activation, it is taking place
> in the (nth period - relative deadline).
>
> Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Reviewed-by: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> Reviewed-by: Juri Lelli <juri.lelli@xxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxx>
> Cc: Tommaso Cucinotta <tommaso.cucinotta@xxxxxxxx>
> Cc: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mike Galbraith <efault@xxxxxx>
> Cc: Romulo Silva de Oliveira <romulo.deoliveira@xxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> kernel/sched/deadline.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 27737f3..3e3caae 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -505,10 +505,15 @@ static void update_dl_entity(struct sched_dl_entity *dl_se,
> }
> }
>
> +static inline u64 dl_next_period(struct sched_dl_entity *dl_se)
> +{
> + return dl_se->deadline - dl_se->dl_deadline + dl_se->dl_period;
> +}
> +
> /*
> * If the entity depleted all its runtime, and if we want it to sleep
> * while waiting for some new execution time to become available, we
> - * set the bandwidth enforcement timer to the replenishment instant
> + * set the bandwidth replenishment timer to the replenishment instant
> * and try to activate it.
> *
> * Notice that it is important for the caller to know if the timer
> @@ -530,7 +535,7 @@ static int start_dl_timer(struct task_struct *p)
> * that it is actually coming from rq->clock and not from
> * hrtimer's time base reading.
> */
> - act = ns_to_ktime(dl_se->deadline);
> + act = ns_to_ktime(dl_next_period(dl_se));
> now = hrtimer_cb_get_time(timer);
> delta = ktime_to_ns(now) - rq_clock(rq);
> act = ktime_add_ns(act, delta);
> --
> 2.9.3
>