From: Luca Abeni <luca.abeni@unitn.it>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@gmail.com>,
Dario Faggioli <raistlin@linux.it>,
linux-kernel@vger.kernel.org, luca.abeni@unitn.it
Subject: [PATCH 2/2] Avoid double-accounting in case of missed deadlines
Date: Wed, 17 Dec 2014 11:50:32 +0100
Message-ID: <1418813432-20797-3-git-send-email-luca.abeni@unitn.it> (raw)
In-Reply-To: <1418813432-20797-1-git-send-email-luca.abeni@unitn.it>
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>
---
kernel/sched/deadline.c | 19 +------------------
1 file changed, 1 insertion(+), 18 deletions(-)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 55af498..b52092f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -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);
--
1.9.1
next prev parent reply index
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-12-17 10:50 [PATCH 0/2] SCHED_DEADLINE fixes Luca Abeni
2014-12-17 10:50 ` [PATCH 1/2] Fix migration of SCHED_DEADLINE tasks Luca Abeni
2015-01-09 12:33 ` [tip:sched/urgent] sched/deadline: " tip-bot for Luca Abeni
2014-12-17 10:50 ` Luca Abeni [this message]
2015-01-09 12:34 ` [tip:sched/urgent] sched/deadline: Avoid double-accounting in case of missed deadlines tip-bot for Luca Abeni
2014-12-17 12:49 ` [PATCH 0/2] SCHED_DEADLINE fixes Juri Lelli
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