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Date: Wed, 05 Nov 2014 18:35:58 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Juri Lelli <juri.lelli@....com>,
Wanpeng Li <wanpeng.li@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
CC: Kirill Tkhai <ktkhai@...allels.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2] sched/deadline: support dl task migration during
cpu hotplug
Hi Juri,
On 14/11/5 下午6:08, Juri Lelli wrote:
> Hi,
>
> On 05/11/14 08:51, 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 fix it by push the task to another cpu in dl_task_timer() if
>> rq is offline.
>>
>> Note: dl task can be migrated successfully if rq is offline currently, however,
>> I'm still not sure why task_rq(task)->rd->span just include the cpu which the dl
>> task previous running on, so cpu_active_mask is used in the patch.
>>
>> Peterz, Juri?
>>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
>> ---
>> v1 -> v2:
>> * push the task to another cpu in dl_task_timer() if rq is offline.
>>
>> kernel/sched/deadline.c | 39 ++++++++++++++++++++++++++++++++++++---
>> 1 file changed, 36 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index 04c2cbb..233e482 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 +539,39 @@ 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 (!rq->online) {
>> + struct rq *later_rq = NULL;
>> +
>> + /* We will release rq lock */
>> + get_task_struct(p);
>> +
>> + raw_spin_unlock(&rq->lock);
>> +
>> + later_rq = find_lock_later_rq(p, rq);
>> +
>> + if (!later_rq) {
>> + put_task_struct(p);
>> + goto out;
>> + }
>> +
>> + 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);
>> +
>> + put_task_struct(p);
>> +
>> + goto out;
>> + }
>> +
>> if (task_on_rq_queued(p)) {
>> enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>> if (dl_task(rq->curr))
>> @@ -555,7 +589,7 @@ again:
>> }
>> unlock:
>> raw_spin_unlock(&rq->lock);
>> -
>> +out:
>> return HRTIMER_NORESTART;
>> }
>>
>> @@ -1182,8 +1216,7 @@ 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);
> I fear this breaks what I lately fixed in commit 91ec6778ec4f
> ("sched/deadline: Fix inter- exclusive cpusets migrations"), as
As I mentioned in the patch description:
Note: dl task can be migrated successfully if rq is offline currently,
however, I'm still not sure why
task_rq(task)->rd->span just include the cpu which the dl task previous
running on, so cpu_active_mask is used in the patch.
Any explantion after your test is a great approciated. ;-)
> we first have to consider exclusive cpusets topology in looking
> for a cpu. But, I'd have to test this to see if I'm right, and
> I'll try to do it soon.
Thanks for your help. ;-)
Regards,
Wanpeng Li
>
> Thanks,
>
> - Juri
>
>> 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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