* [PATCH 0/2] SCHED_DEADLINE fixes
@ 2014-12-17 10:50 Luca Abeni
2014-12-17 10:50 ` [PATCH 1/2] Fix migration of SCHED_DEADLINE tasks Luca Abeni
` (2 more replies)
0 siblings, 3 replies; 6+ messages in thread
From: Luca Abeni @ 2014-12-17 10:50 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Ingo Molnar, Juri Lelli, Dario Faggioli, linux-kernel, luca.abeni
Hi all,
I noticed some discrepancies between the schedule produced
by SCHED_DEADLINE and the expectations from real-time
scheduling theory. After some investigations, it turned out
that such discrepancies are due to two bugs in deadline.c,
which are particularly visible when using global scheduling
on multiple CPUs (see the two patches for more details).
I think the first bug (fixed in patch 0001) is particularly
critical, because it causes a violation of the SCHED_DEADLINE
guarantee (if the total load is smaller than the number of
CPUs, there is an upper bound for the response times. This is
a well known property for global EDF, but is not respected by
SCHED_DEADLINE - see patch 0001 for more details).
The second patch is IMHO also important, but less critical.
Luca Abeni (2):
Fix migration of SCHED_DEADLINE tasks
Avoid double-accounting in case of missed deadlines
kernel/sched/deadline.c | 25 ++++---------------------
1 file changed, 4 insertions(+), 21 deletions(-)
--
1.9.1
^ permalink raw reply [flat|nested] 6+ messages in thread* [PATCH 1/2] Fix migration of SCHED_DEADLINE tasks 2014-12-17 10:50 [PATCH 0/2] SCHED_DEADLINE fixes Luca Abeni @ 2014-12-17 10:50 ` Luca Abeni 2015-01-09 12:33 ` [tip:sched/urgent] sched/deadline: " tip-bot for Luca Abeni 2014-12-17 10:50 ` [PATCH 2/2] Avoid double-accounting in case of missed deadlines Luca Abeni 2014-12-17 12:49 ` [PATCH 0/2] SCHED_DEADLINE fixes Juri Lelli 2 siblings, 1 reply; 6+ messages in thread From: Luca Abeni @ 2014-12-17 10:50 UTC (permalink / raw) To: Peter Zijlstra Cc: Ingo Molnar, Juri Lelli, Dario Faggioli, linux-kernel, luca.abeni According to global EDF, tasks should be migrated between runqueues without checking if their scheduling deadlines and runtimes are valid. However, SCHED_DEADLINE currently performs such a check: a migration happens doing deactivate_task(rq, next_task, 0); set_task_cpu(next_task, later_rq->cpu); activate_task(later_rq, next_task, 0); which ends up calling dequeue_task_dl(), setting the new CPU, and then calling enqueue_task_dl(). enqueue_task_dl() then calls enqueue_dl_entity(), which calls update_dl_entity(), which can modify scheduling deadline and runtime, breaking global EDF scheduling. As a result, some of the properties of global EDF are not respected: for example, a taskset {(30, 80), (40, 80), (120, 170)} scheduled on two cores can have unbounded response times for the third task even if 30/80+40/80+120/170 = 1.5809 < 2 This can be fixed by invoking update_dl_entity() only in case of wakeup, or if this is a new SCHED_DEADLINE task. Signed-off-by: Luca Abeni <luca.abeni@unitn.it> --- kernel/sched/deadline.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index e5db8c6..55af498 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -826,10 +826,10 @@ enqueue_dl_entity(struct sched_dl_entity *dl_se, * parameters of the task might need updating. Otherwise, * we want a replenishment of its runtime. */ - if (!dl_se->dl_new && flags & ENQUEUE_REPLENISH) - replenish_dl_entity(dl_se, pi_se); - else + if (dl_se->dl_new || flags & ENQUEUE_WAKEUP) update_dl_entity(dl_se, pi_se); + else if (flags & ENQUEUE_REPLENISH) + replenish_dl_entity(dl_se, pi_se); __enqueue_dl_entity(dl_se); } -- 1.9.1 ^ permalink raw reply related [flat|nested] 6+ messages in thread
* [tip:sched/urgent] sched/deadline: Fix migration of SCHED_DEADLINE tasks 2014-12-17 10:50 ` [PATCH 1/2] Fix migration of SCHED_DEADLINE tasks Luca Abeni @ 2015-01-09 12:33 ` tip-bot for Luca Abeni 0 siblings, 0 replies; 6+ messages in thread From: tip-bot for Luca Abeni @ 2015-01-09 12:33 UTC (permalink / raw) To: linux-tip-commits Cc: hpa, mingo, torvalds, peterz, tglx, luca.abeni, linux-kernel, juri.lelli, raistlin, stable Commit-ID: 6a503c3be937d275113b702e0421e5b0720abe8a Gitweb: http://git.kernel.org/tip/6a503c3be937d275113b702e0421e5b0720abe8a Author: Luca Abeni <luca.abeni@unitn.it> AuthorDate: Wed, 17 Dec 2014 11:50:31 +0100 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Fri, 9 Jan 2015 11:18:56 +0100 sched/deadline: Fix migration of SCHED_DEADLINE tasks According to global EDF, tasks should be migrated between runqueues without checking if their scheduling deadlines and runtimes are valid. However, SCHED_DEADLINE currently performs such a check: a migration happens doing: deactivate_task(rq, next_task, 0); set_task_cpu(next_task, later_rq->cpu); activate_task(later_rq, next_task, 0); which ends up calling dequeue_task_dl(), setting the new CPU, and then calling enqueue_task_dl(). enqueue_task_dl() then calls enqueue_dl_entity(), which calls update_dl_entity(), which can modify scheduling deadline and runtime, breaking global EDF scheduling. As a result, some of the properties of global EDF are not respected: for example, a taskset {(30, 80), (40, 80), (120, 170)} scheduled on two cores can have unbounded response times for the third task even if 30/80+40/80+120/170 = 1.5809 < 2 This can be fixed by invoking update_dl_entity() only in case of wakeup, or if this is a new SCHED_DEADLINE task. 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/1418813432-20797-2-git-send-email-luca.abeni@unitn.it Signed-off-by: Ingo Molnar <mingo@kernel.org> --- kernel/sched/deadline.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index e5db8c6..55af498 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -826,10 +826,10 @@ enqueue_dl_entity(struct sched_dl_entity *dl_se, * parameters of the task might need updating. Otherwise, * we want a replenishment of its runtime. */ - if (!dl_se->dl_new && flags & ENQUEUE_REPLENISH) - replenish_dl_entity(dl_se, pi_se); - else + if (dl_se->dl_new || flags & ENQUEUE_WAKEUP) update_dl_entity(dl_se, pi_se); + else if (flags & ENQUEUE_REPLENISH) + replenish_dl_entity(dl_se, pi_se); __enqueue_dl_entity(dl_se); } ^ permalink raw reply related [flat|nested] 6+ messages in thread
* [PATCH 2/2] Avoid double-accounting in case of missed deadlines 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 @ 2014-12-17 10:50 ` Luca Abeni 2015-01-09 12:34 ` [tip:sched/urgent] sched/deadline: " tip-bot for Luca Abeni 2014-12-17 12:49 ` [PATCH 0/2] SCHED_DEADLINE fixes Juri Lelli 2 siblings, 1 reply; 6+ messages in thread From: Luca Abeni @ 2014-12-17 10:50 UTC (permalink / raw) To: Peter Zijlstra Cc: Ingo Molnar, Juri Lelli, Dario Faggioli, linux-kernel, luca.abeni 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 ^ permalink raw reply related [flat|nested] 6+ messages in thread
* [tip:sched/urgent] sched/deadline: Avoid double-accounting in case of missed deadlines 2014-12-17 10:50 ` [PATCH 2/2] Avoid double-accounting in case of missed deadlines Luca Abeni @ 2015-01-09 12:34 ` tip-bot for Luca Abeni 0 siblings, 0 replies; 6+ messages in thread From: tip-bot for Luca Abeni @ 2015-01-09 12:34 UTC (permalink / raw) To: linux-tip-commits Cc: tglx, linux-kernel, stable, hpa, juri.lelli, raistlin, torvalds, peterz, mingo, luca.abeni Commit-ID: 269ad8015a6b2bb1cf9e684da4921eb6fa0a0c88 Gitweb: http://git.kernel.org/tip/269ad8015a6b2bb1cf9e684da4921eb6fa0a0c88 Author: Luca Abeni <luca.abeni@unitn.it> AuthorDate: Wed, 17 Dec 2014 11:50:32 +0100 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Fri, 9 Jan 2015 11:18:57 +0100 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> --- 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); ^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH 0/2] SCHED_DEADLINE fixes 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 2014-12-17 10:50 ` [PATCH 2/2] Avoid double-accounting in case of missed deadlines Luca Abeni @ 2014-12-17 12:49 ` Juri Lelli 2 siblings, 0 replies; 6+ messages in thread From: Juri Lelli @ 2014-12-17 12:49 UTC (permalink / raw) To: Luca Abeni, Peter Zijlstra Cc: Ingo Molnar, Juri Lelli, Dario Faggioli, linux-kernel Hi Luca, On 17/12/14 10:50, Luca Abeni wrote: > Hi all, > > I noticed some discrepancies between the schedule produced > by SCHED_DEADLINE and the expectations from real-time > scheduling theory. After some investigations, it turned out > that such discrepancies are due to two bugs in deadline.c, > which are particularly visible when using global scheduling > on multiple CPUs (see the two patches for more details). > > I think the first bug (fixed in patch 0001) is particularly > critical, because it causes a violation of the SCHED_DEADLINE > guarantee (if the total load is smaller than the number of > CPUs, there is an upper bound for the response times. This is > a well known property for global EDF, but is not respected by > SCHED_DEADLINE - see patch 0001 for more details). > The second patch is IMHO also important, but less critical. > I already reviewed and tested them. They looks ok and are important fixes. ACK for both. :) Thanks a lot! - Juri > Luca Abeni (2): > Fix migration of SCHED_DEADLINE tasks > Avoid double-accounting in case of missed deadlines > > kernel/sched/deadline.c | 25 ++++--------------------- > 1 file changed, 4 insertions(+), 21 deletions(-) > ^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2015-01-09 12:34 UTC | newest] Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 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 ` [PATCH 2/2] Avoid double-accounting in case of missed deadlines Luca Abeni 2015-01-09 12:34 ` [tip:sched/urgent] sched/deadline: " tip-bot for Luca Abeni 2014-12-17 12:49 ` [PATCH 0/2] SCHED_DEADLINE fixes Juri Lelli
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