Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753616AbdCFPwD (ORCPT ); Mon, 6 Mar 2017 10:52:03 -0500 Received: from mail.santannapisa.it ([193.205.80.99]:38574 "EHLO mail.santannapisa.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753129AbdCFPvx (ORCPT ); Mon, 6 Mar 2017 10:51:53 -0500 Date: Mon, 6 Mar 2017 16:51:39 +0100 From: Luca Abeni To: Daniel Bristot de Oliveira Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Juri Lelli , Tommaso Cucinotta , Steven Rostedt , Mike Galbraith , Romulo Silva de Oliveira Subject: Re: [PATCH V4 2/3] sched/deadline: Throttle a constrained deadline task activated after the deadline Message-ID: <20170306165139.3fb303fc@luca> In-Reply-To: References: X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6048 Lines: 177 On Thu, 2 Mar 2017 15:10:58 +0100 Daniel Bristot de Oliveira wrote: > During the activation, CBS checks if it can reuse the current task's > runtime and period. If the deadline of the task is in the past, CBS > cannot use the runtime, and so it replenishes the task. This rule > works fine for implicit deadline tasks (deadline == period), and the > CBS was designed for implicit deadline tasks. However, a task with > constrained deadline (deadine < period) might be awakened after the > deadline, but before the next period. In this case, replenishing the > task would allow it to run for runtime / deadline. As in this case > deadline < period, CBS enables a task to run for more than the > runtime / period. In a very loaded system, this can cause a domino > effect, making other tasks miss their deadlines. > > To avoid this problem, in the activation of a constrained deadline > task after the deadline but before the next period, throttle the > task and set the replenishing timer to the begin of the next period, > unless it is boosted. [...] I agree with Daniel that the current code is broken here... And I think this patch is a reasonable solution (maybe we can improve it later, but I think a fix for this issue should go in soon). So, Reviewed-by: Luca Abeni Thanks, Luca > > Reproducer: > > --------------- %< --------------- > int main (int argc, char **argv) > { > int ret; > int flags = 0; > unsigned long l = 0; > struct timespec ts; > struct sched_attr attr; > > memset(&attr, 0, sizeof(attr)); > attr.size = sizeof(attr); > > attr.sched_policy = SCHED_DEADLINE; > attr.sched_runtime = 2 * 1000 * 1000; /* 2 ms > */ attr.sched_deadline = 2 * 1000 * 1000; /* 2 ms */ > attr.sched_period = 2 * 1000 * 1000 * 1000; /* 2 s */ > > ts.tv_sec = 0; > ts.tv_nsec = 2000 * 1000; /* 2 ms */ > > ret = sched_setattr(0, &attr, flags); > > if (ret < 0) { > perror("sched_setattr"); > exit(-1); > } > > for(;;) { > /* XXX: you may need to adjust the loop */ > for (l = 0; l < 150000; l++); > /* > * The ideia is to go to sleep right before the > deadline > * and then wake up before the next period to receive > * a new replenishment. > */ > nanosleep(&ts, NULL); > } > > exit(0); > } > --------------- >% --------------- > > On my box, this reproducer uses almost 50% of the CPU time, which is > obviously wrong for a task with 2/2000 reservation. > > Signed-off-by: Daniel Bristot de Oliveira > Cc: Ingo Molnar > Cc: Peter Zijlstra > Cc: Juri Lelli > Cc: Tommaso Cucinotta > Cc: Luca Abeni > Cc: Steven Rostedt > Cc: Mike Galbraith > Cc: Romulo Silva de Oliveira > Cc: Daniel Bristot de Oliveira > Cc: linux-kernel@vger.kernel.org > --- > kernel/sched/deadline.c | 45 > +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 45 > insertions(+) > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index 3e3caae..b669f7f 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -694,6 +694,37 @@ void init_dl_task_timer(struct sched_dl_entity > *dl_se) timer->function = dl_task_timer; > } > > +/* > + * During the activation, CBS checks if it can reuse the current > task's > + * runtime and period. If the deadline of the task is in the past, > CBS > + * cannot use the runtime, and so it replenishes the task. This rule > + * works fine for implicit deadline tasks (deadline == period), and > the > + * CBS was designed for implicit deadline tasks. However, a task with > + * constrained deadline (deadine < period) might be awakened after > the > + * deadline, but before the next period. In this case, replenishing > the > + * task would allow it to run for runtime / deadline. As in this case > + * deadline < period, CBS enables a task to run for more than the > + * runtime / period. In a very loaded system, this can cause a domino > + * effect, making other tasks miss their deadlines. > + * > + * To avoid this problem, in the activation of a constrained deadline > + * task after the deadline but before the next period, throttle the > + * task and set the replenishing timer to the begin of the next > period, > + * unless it is boosted. > + */ > +static inline void dl_check_constrained_dl(struct sched_dl_entity > *dl_se) +{ > + struct task_struct *p = dl_task_of(dl_se); > + struct rq *rq = rq_of_dl_rq(dl_rq_of_se(dl_se)); > + > + if (dl_time_before(dl_se->deadline, rq_clock(rq)) && > + dl_time_before(rq_clock(rq), dl_next_period(dl_se))) { > + if (unlikely(dl_se->dl_boosted > || !start_dl_timer(p))) > + return; > + dl_se->dl_throttled = 1; > + } > +} > + > static > int dl_runtime_exceeded(struct sched_dl_entity *dl_se) > { > @@ -927,6 +958,11 @@ static void dequeue_dl_entity(struct > sched_dl_entity *dl_se) __dequeue_dl_entity(dl_se); > } > > +static inline bool dl_is_constrained(struct sched_dl_entity *dl_se) > +{ > + return dl_se->dl_deadline < dl_se->dl_period; > +} > + > static void enqueue_task_dl(struct rq *rq, struct task_struct *p, > int flags) { > struct task_struct *pi_task = rt_mutex_get_top_task(p); > @@ -953,6 +989,15 @@ static void enqueue_task_dl(struct rq *rq, > struct task_struct *p, int flags) } > > /* > + * Check if a constrained deadline task was activated > + * after the deadline but before the next period. > + * If that is the case, the task will be throttled and > + * the replenishment timer will be set to the next period. > + */ > + if (!p->dl.dl_throttled && dl_is_constrained(&p->dl)) > + dl_check_constrained_dl(&p->dl); > + > + /* > * If p is throttled, we do nothing. In fact, if it exhausted > * its budget it needs a replenishment and, since it now is > on > * its rq, the bandwidth timer callback (which clearly has > not