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SubjectRe: [PATCH v5] sched/deadline: support dl task migration during cpu hotplug
Hi Kirill,
On Wed, Nov 12, 2014 at 07:27:06PM +0300, Kirill Tkhai wrote:
>В Ср, 12/11/2014 в 09:06 +0800, Wanpeng Li пишет:
>> 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.
>>
>> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
>> ---
>> 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 | 43 +++++++++++++++++++++++++++++++++++++++++--
>> 1 file changed, 41 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index f3d7776..7c31906 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -487,6 +487,7 @@ static int start_dl_timer(struct sched_dl_entity *dl_se, bool boosted)
>> return hrtimer_active(&dl_se->dl_timer);
>> }
>>
>> +static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
>> /*
>> * This is the bandwidth enforcement timer callback. If here, we know
>> * a task is not on its dl_rq, since the fact that the timer was running
>> @@ -538,6 +539,43 @@ again:
>> update_rq_clock(rq);
>> dl_se->dl_throttled = 0;
>> dl_se->dl_yielded = 0;
>> +
>> + /*
>> + * 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)) {
>> + struct rq *later_rq = NULL;
>> +
>> + later_rq = find_lock_later_rq(p, rq);
>> +
>> + if (!later_rq) {
>> + int cpu;
>> +
>> + /*
>> + * If cannot preempt any rq, fallback to pick any
>> + * online cpu.
>> + */
>> + cpu = cpumask_any_and(cpu_active_mask,
>> + tsk_cpus_allowed(p));
>> + if (cpu >= nr_cpu_ids) {
>> + pr_warn("fail to find any online cpu and task will never come back\n");
>> + goto unlock;
>> + }
>> + later_rq = cpu_rq(cpu);
>
>later_rq is not locked here, but you activate p on it and you do unlock below.

Great catch! How about add double_lock_balance(rq, later_rq); here?

Regards,
Wanpeng Li

>
>> + }
>> +
>> + deactivate_task(rq, p, 0);
>> + set_task_cpu(p, later_rq->cpu);
>> + activate_task(later_rq, p, 0);
>
> ^^^^^
>
>> +
>> + resched_curr(later_rq);
>> +
>> + double_unlock_balance(rq, later_rq);
>
> ^^^^^^
>
>> +
>> + goto unlock;
>> + }
>> +
>> if (task_on_rq_queued(p)) {
>> enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>> if (dl_task(rq->curr))
>> @@ -1185,8 +1223,9 @@ static int find_later_rq(struct task_struct *task)
>> * We have to consider system topology and task affinity
>> * first, then we can look for a suitable cpu.
>> */
>> - cpumask_copy(later_mask, task_rq(task)->rd->span);
>> - cpumask_and(later_mask, later_mask, cpu_active_mask);
>> + cpumask_copy(later_mask, cpu_active_mask);
>> + if (likely(task_rq(task)->online))
>> + cpumask_and(later_mask, later_mask, task_rq(task)->rd->span);
>> cpumask_and(later_mask, later_mask, &task->cpus_allowed);
>> best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
>> task, later_mask);
>
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