Commit 269ad801 authored Dec 17, 2014 by Luca Abeni Committed by Ingo Molnar Jan 09, 2015

sched/deadline: Avoid double-accounting in case of missed deadlines



The dl_runtime_exceeded() function is supposed to ckeck if
a SCHED_DEADLINE task must be throttled, by checking if its
current runtime is <= 0. However, it also checks if the
scheduling deadline has been missed (the current time is
larger than the current scheduling deadline), further
decreasing the runtime if this happens.
This "double accounting" is wrong:

- In case of partitioned scheduling (or single CPU), this
  happens if task_tick_dl() has been called later than expected
  (due to small HZ values). In this case, the current runtime is
  also negative, and replenish_dl_entity() can take care of the
  deadline miss by recharging the current runtime to a value smaller
  than dl_runtime

- In case of global scheduling on multiple CPUs, scheduling
  deadlines can be missed even if the task did not consume more
  runtime than expected, hence penalizing the task is wrong

This patch fix this problem by throttling a SCHED_DEADLINE task
only when its runtime becomes negative, and not modifying the runtime

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@gmail.com>
Cc: <stable@vger.kernel.org>
Cc: Dario Faggioli <raistlin@linux.it>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1418813432-20797-3-git-send-email-luca.abeni@unitn.it


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 6a503c3b

kernel/sched/deadline.c

+1 −18

Original line number	Diff line number	Diff line
		@@ -570,24 +570,7 @@ void init_dl_task_timer(struct sched_dl_entity *dl_se)
		static
		int dl_runtime_exceeded(struct rq rq, struct sched_dl_entity dl_se)
		{
		int dmiss = dl_time_before(dl_se->deadline, rq_clock(rq));
		int rorun = dl_se->runtime <= 0;

		if (!rorun && !dmiss)
		return 0;

		/*
		* If we are beyond our current deadline and we are still
		* executing, then we have already used some of the runtime of
		* the next instance. Thus, if we do not account that, we are
		* stealing bandwidth from the system at each deadline miss!
		*/
		if (dmiss) {
		dl_se->runtime = rorun ? dl_se->runtime : 0;
		dl_se->runtime -= rq_clock(rq) - dl_se->deadline;
		}

		return 1;
		return (dl_se->runtime <= 0);
		}

		extern bool sched_rt_bandwidth_account(struct rt_rq *rt_rq);

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