Re: [PATCH] sched/deadline: Fix a bug in dl_overflow()

Xunlei Pang
2016-04-15T08:13:50Z

On 2016/04/15 at 15:07, Juri Lelli wrote:
> [+Luca]
>
> Hi,
>
> On 14/04/16 20:19, Xunlei Pang wrote:
>> I got a minus(very big) dl_b->total_bw during my deadline tests.
>>
>> # grep dl /proc/sched_debug
>> dl_rq[0]:
>> .dl_nr_running : 0
>> .dl_bw->bw : 996147
>> .dl_bw->total_bw : -222297900
>>
>> Something unusual must have happened.
>>
>> After some digging, I finally noticed that when changing a deadline
>> task to normal(cfs), and changing it back to deadline immediately,
>> after it died, we will got the wrong dl_bw->total_bw.
>>
>> The root cause is in dl_overflow(), it has:
>> if (new_bw == p->dl.dl_bw)
>> return 0;
>>
>> 1) When a deadline task is changed to !deadline task, it will start
>> dl timer in switched_from_dl(), and retain previous deadline parameter
>> till the timer expires.
>> 2) If we change it back to deadline with the same bandwidth parameter
>> before the timer expires, as it keeps the old bandwidth although it
>> is not a deadline task. dl_overflow() simply returns success without
>> updating the right data, and got the wrong dl_bw->total_bw.
>>
>> The solution is simple, if @p is not deadline, don't return.
>>
>> Signed-off-by: Xunlei Pang <xlpang@redhat.com>
>> ---
>> kernel/sched/core.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 4a2c79d..5988fee 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2378,7 +2378,8 @@ static int dl_overflow(struct task_struct *p, int policy,
>> u64 new_bw = dl_policy(policy) ? to_ratio(period, runtime) : 0;
>> int cpus, err = -1;
>>
>> - if (new_bw == p->dl.dl_bw)
>> + /* !deadline task may carry old deadline bandwidth */
>> + if (new_bw == p->dl.dl_bw && task_has_dl_policy(p))
> Right. I got the same patch that I believe Luca is be already using for
> his tests (and he also put together the changelog). I never managed to
> send it out, sorry about that. We can take yours, mine follows just in
> case we want to take something from the changelog or we want to reverse
> the if condition.

Ah, exactly the same issue. Thanks for your feedback :-)

Regards,
Xunlei

>
> Thanks,
>
> - Juri
>
> --->8---
>
> From 4bf38111bd9383035e03d3dc3d42011aaa9e26e7 Mon Sep 17 00:00:00 2001
> From: Juri Lelli <juri.lelli@arm.com>
> Date: Thu, 25 Feb 2016 15:50:42 +0100
> Subject: [PATCH 2/3] fix a bug in the -deadline utilization tracking mechanism
>
> Currently, a task doing
> while(1) {
> switch to SCHED_DEADLINE
> switch to SCHED_OTHER
> }
> brings dl_b->total_bw below 0.
> This happens because when the task switches back from SCHED_DEADLINE
> to SCHED_OTHER, switched_from_dl() does not clear its deadline
> parameters (they will be cleared by the deadline timer when it fires).
> But dl_overflow() removes its utilization from dl_b->total_bw.
> When the task switches back to SCHED_DEADLINE, the
> if (new_bw == p->dl.dl_bw)
> check in dl_overflow() prevents __dl_add() from being called, and
> so when the task switches back to SCHED_OTHER dl_b->total_bw becomes
> negative.
> This patch changes the check so that if the task is switching from
> SCHED_OTHER to SCHED_DEADLINE __dl_add() is correctly invoked.
> ---
> kernel/sched/core.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9503d59..d59fa20 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2432,7 +2432,7 @@ static int dl_overflow(struct task_struct *p, int policy,
> u64 new_bw = dl_policy(policy) ? to_ratio(period, runtime) : 0;
> int cpus, err = -1;
>
> - if (new_bw == p->dl.dl_bw)
> + if (task_has_dl_policy(p) && new_bw == p->dl.dl_bw)
> return 0;
>
> /*