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Date: Tue, 11 Nov 2014 16:09:14 +0300
From: Kirill Tkhai <ktkhai@...allels.com>
To: Wanpeng Li <kernellwp@...il.com>
CC: Wanpeng Li <wanpeng.li@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@....com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] sched/deadline: support dl task migration during cpu
hotplug
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.
>
> >
> >> +
> >> + 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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