linux-kernel - Re: [PATCH] sched/deadline: Fix a bug in dl

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Date:	Fri, 15 Apr 2016 16:13:41 +0800
From:	Xunlei Pang <xpang@...hat.com>
To:	Juri Lelli <juri.lelli@....com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	luca abeni <luca.abeni@...tn.it>
Subject: Re: [PATCH] sched/deadline: Fix a bug in dl_overflow()

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@...hat.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@....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;
>
>  	/*