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Date: Wed, 12 Nov 2014 07:53:20 +0800
From: Wanpeng Li <wanpeng.li@...ux.intel.com>
To: Kirill Tkhai <ktkhai@...allels.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org,
Wanpeng Li <wanpeng.li@...ux.intel.com>
Subject: Re: [PATCH v4] sched/deadline: support dl task migration during cpu
hotplug
Hi Kirill,
On Tue, Nov 11, 2014 at 04:09:14PM +0300, Kirill Tkhai wrote:
>В Вт, 11/11/2014 в 21:07 +0800, Wanpeng Li пишет:
>> Hi Kirill,
>> On 11/11/14, 7:10 PM, Kirill Tkhai wrote:
>> > В Вт, 11/11/2014 в 10:30 +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 fix it by push the task to another cpu in dl_task_timer() if
>> >> rq is offline.
>> >>
>> >> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
>> > I'm still thinking we don't have to guarantee any "deadlines" during cpu hotplug...
>> > But, if speaking about this way:
>> >
>> >> ---
>> >> 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 | 51 ++++++++++++++++++++++++++++++++++++++++++++++---
>> >> 1 file changed, 48 insertions(+), 3 deletions(-)
>> >>
>> >> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> >> index 00324af..e0fbba4 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 +539,46 @@ 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;
>> >> +
>> >> + raw_spin_unlock(&rq->lock);
>> >> +
>> >> + later_rq = find_lock_later_rq(p, rq);
>> > find_lock_later_rq() expects that rq is locked.
>> >
>> > The comment near its head confuses a reader. It locks newly found rq.
>>
>> Sorry for my bad, what's you think should be changed?
>
>raw_spin_unlock(&rq->lock) is wrong here. It's not need.
>
The machine down after remove this.
Regards,
Wanpeng Li
>>
>> >
>> >> +
>> >> + if (!later_rq) {
>> >> + int cpu;
>> >> +
>> >> + /*
>> >> + * If cannot preempt any rq, fallback to pick any
>> >> + * online cpu.
>> >> + */
>> >> + for_each_cpu(cpu, tsk_cpus_allowed(p))
>> >> + if (cpu_online(cpu))
>> >> + later_rq = cpu_rq(cpu);
>> >> + if (!later_rq) {
>> >> + pr_warn("fail to find any online and task "
>> >> + "will never come back to us\n");
>> >> + 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);
>> > double_unlock_balance() unlocks later_rq only.
>> >
>> >> +
>> >> + goto out;
>> >> + }
>> >> +
>> >> if (task_on_rq_queued(p)) {
>> >> enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>> >> if (dl_task(rq->curr))
>> >> @@ -555,7 +596,7 @@ again:
>> >> }
>> >> unlock:
>> >> raw_spin_unlock(&rq->lock);
>> >> -
>> >> +out:
>> >> return HRTIMER_NORESTART;
>> >> }
>> >>
>> >> @@ -1185,8 +1226,12 @@ 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);
>> >> + if (likely(task_rq(task)->online)) {
>> >> + cpumask_copy(later_mask, task_rq(task)->rd->span);
>> >> + cpumask_and(later_mask, later_mask, cpu_active_mask);
>> >> + } else
>> >> + /* for offline cpus we have a singleton rd */
>> >> + cpumask_copy(later_mask, cpu_active_mask);
>> >> 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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>>
>
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