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Date: Thu, 26 Mar 2015 11:14:37 +0000
From: Juri Lelli <juri.lelli@....com>
To: Wanpeng Li <wanpeng.li@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v13] sched/deadline: support dl task migration during
cpu hotplug
Hi,
On 25/03/2015 23:47, Wanpeng Li wrote:
> I observe that dl task can't be migrated to other cpus during cpu hotplug,
> in addition, task may/may not be running again if cpu is added back. The
> root cause which I found is that dl task will be throtted and removed from
> dl rq after comsuming all budget, which leads to stop task can't pick it up
> from dl rq and migrate to other cpus during hotplug.
>
> The method to reproduce:
> schedtool -E -t 50000:100000 -e ./test
> Actually test is just a simple for loop. Then observe which cpu the test
> task is on.
> echo 0 > /sys/devices/system/cpu/cpuN/online
>
> This patch adds the dl task migration during cpu hotplug by finding a most
> suitable later deadline rq after dl timer fire if current rq is offline,
> if fail to find a suitable later deadline rq then fallback to any eligible
> online cpu in order that the deadline task will come back to us, and the
> push/pull mechanism should then move it around properly.
>
> Suggested-and-acked-by: Juri Lelli <juri.lelli@....com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
> v12 -> 13:
> * move hotplug stuff to CONFIG_SMP in order to fix the error reported by kbuild test robot
> v11 -> v12:
> * s/WARN_ON/BUG_ON
> v10 -> v11:
> * fix codes comments
> * tsk_cpus_allowed(p) shouldn't be on separate lines
> * introduce a helper function to fold dl task migration during cpu hotplug support
> v9 -> v10:
> * fix the "WARNING: line over 80 characters"
> * handle no admission control
> v8 -> v9:
> * align tsk_cpus_allowed(p) to cpu_active_mask
> * add WARN_ON(1)
> * don't resched_curr if later_rq come from the cpumask_any_and()
> v7 -> v8:
> * remove rd->span related modification since Pang's commit 16b269436b72
> (sched/deadline: Modify cpudl::free_cpus to reflect rd->online) merged
> upstream, which Juri pointed out can handle the exclusive cpusets.
> * rebase
> v6 -> v7:
> * rebase
> v5 -> v6:
> * add double_lock_balance in the fallback path
> v4 -> v5:
> * remove raw_spin_unlock(&rq->lock)
> * cleanup codes, spotted by Peterz
> * cleanup patch description
> v3 -> v4:
> * use tsk_cpus_allowed wrapper
> * fix compile error
> v2 -> v3:
> * don't get_task_struct
> * if cannot preempt any rq, fallback to pick any online cpus
> * use cpu_active_mask as original later_mask if cpu is offline
> v1 -> v2:
> * push the task to another cpu in dl_task_timer() if rq is offline.
>
> kernel/sched/deadline.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 59 insertions(+)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 24c18dc..60b7e90 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -218,6 +218,54 @@ static inline void set_post_schedule(struct rq *rq)
> rq->post_schedule = has_pushable_dl_tasks(rq);
> }
>
> +static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
> +
> +static void dl_task_migration(struct rq *rq, struct task_struct *p)
> +{
> + struct rq *later_rq = NULL;
> + bool fallback = false;
> +
> + later_rq = find_lock_later_rq(p, rq);
> +
> + if (!later_rq) {
> + int cpu;
> +
> + /*
> + * If we cannot preempt any rq, fall back to pick any
> + * online cpu.
> + */
> + fallback = true;
> + cpu = cpumask_any_and(cpu_active_mask, tsk_cpus_allowed(p));
> + if (cpu >= nr_cpu_ids) {
> + if (dl_bandwidth_enabled()) {
> + /*
> + * Fail to find any suitable cpu.
> + * The task will never come back!
> + */
> + BUG_ON(1);
> + return;
> + }
> + /*
> + * If admission control is disabled we
> + * try a little harder to let the task
> + * run.
> + */
> + cpu = cpumask_any(cpu_active_mask);
> + }
> + later_rq = cpu_rq(cpu);
> + double_lock_balance(rq, later_rq);
> + }
> +
> + deactivate_task(rq, p, 0);
> + set_task_cpu(p, later_rq->cpu);
> + activate_task(later_rq, p, ENQUEUE_REPLENISH);
> +
> + if (!fallback)
> + resched_curr(later_rq);
> +
> + double_unlock_balance(rq, later_rq);
> +}
> +
> #else
>
> static inline
> @@ -536,6 +584,17 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
> sched_clock_tick();
> update_rq_clock(rq);
>
> +#ifdef CONFIG_SMP
> + /*
> + * So if we find that the rq the task was on is no longer
> + * available, we need to select a new rq.
> + */
> + if (unlikely(!rq->online)) {
> + dl_task_migration(rq, p);
> + goto unlock;
> + }
> +#endif
> +
> /*
> * If the throttle happened during sched-out; like:
> *
>
I think we could have better naming and the BUG_ON part could be
simplified. I'd propose something like this (sorry, it is based
on one of your previous versions.. I'm a bit struggling keeping
up with the pace ;)).
Thanks!
---
kernel/sched/deadline.c | 20 +++++++++-----------
1 file changed, 9 insertions(+), 11 deletions(-)
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 1820d1b..2b260a9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -518,7 +518,7 @@ static int start_dl_timer(struct sched_dl_entity *dl_se, bool boosted)
static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
-static void dl_task_migration(struct rq *rq, struct task_struct *p)
+static void dl_task_offline_migration(struct rq *rq, struct task_struct *p)
{
struct rq *later_rq = NULL;
bool fallback = false;
@@ -535,14 +535,12 @@ static void dl_task_migration(struct rq *rq, struct task_struct *p)
fallback = true;
cpu = cpumask_any_and(cpu_active_mask, tsk_cpus_allowed(p));
if (cpu >= nr_cpu_ids) {
- if (dl_bandwidth_enabled()) {
- /*
- * Fail to find any suitable cpu.
- * The task will never come back!
- */
- WARN_ON(1);
- return;
- }
+ /*
+ * Fail to find any suitable cpu.
+ * The task will never come back!
+ */
+ BUG_ON(dl_bandwidth_enabled());
+
/*
* If admission control is disabled we
* try a little harder to let the task
@@ -609,11 +607,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
update_rq_clock(rq);
/*
- * So if we find that the rq the task was on is no longer
+ * If we find that the rq the task was on is no longer
* available, we need to select a new rq.
*/
if (unlikely(!rq->online)) {
- dl_task_migration(rq, p);
+ dl_task_offline_migration(rq, p);
goto unlock;
}
--
2.3.0
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