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Subject: Re: WARN ON at kernel/sched/deadline.c task_non_contending
To:     luca abeni <luca.abeni@santannapisa.it>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Li Bin <huawei.libin@huawei.com>,
        "Xiexiuqi (Xie XiuQi)" <xiexiuqi@huawei.com>, <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>
References: <dfb46874-3657-71c6-e068-9413ffe3f82e@huawei.com>
 <20190313154948.773427d6@luca64>
From:   "chengjian (D)" <cj.chengjian@huawei.com>
Message-ID: <0032cd53-05b4-002a-6860-62b51e9e8299@huawei.com>
Date:   Fri, 15 Mar 2019 08:43:00 +0800
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On 2019/3/13 22:49, luca abeni wrote:
> Hi,
>
> After looking at the patch a little bit more and running some tests,
> I suspect this solution might be racy:
> when the timer is already active, (and hrtimer_start() fails), it
> relies on its handler to decrease the running bw (by setting
> dl_non_contending to 1)... But inactive_task_timer() might have
> already checked dl_non_contending, finding it equal to 0 (so, it
> ends up doing nothing and the running bw is not decreased).
>
>
> So, I would prefer a different solution. I think this patch should work:
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 6a73e41a2016..43901fa3f269 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -252,7 +252,6 @@ static void task_non_contending(struct task_struct *p)
>   	if (dl_entity_is_special(dl_se))
>   		return;
>   
> -	WARN_ON(hrtimer_active(&dl_se->inactive_timer));
>   	WARN_ON(dl_se->dl_non_contending);
>   
>   	zerolag_time = dl_se->deadline -
> @@ -269,7 +268,7 @@ static void task_non_contending(struct task_struct *p)
>   	 * If the "0-lag time" already passed, decrease the active
>   	 * utilization now, instead of starting a timer
>   	 */
> -	if (zerolag_time < 0) {
> +	if ((zerolag_time < 0) || hrtimer_active(&dl_se->inactive_timer)) {
>   		if (dl_task(p))
>   			sub_running_bw(dl_se, dl_rq);
>   		if (!dl_task(p) || p->state == TASK_DEAD) {
>
>
> The idea is that if the timer is active, we leave dl_non_contending set to
> 0 (so that the timer handler does nothing), and we immediately decrease the
> running bw.
> I think this is OK, because this situation can happen only if the task
> blocks, wakes up while the timer handler is running, and then immediately
> blocks again - while the timer handler is still running. So, the "zero lag
> time" cannot be too much in the future.
>
>
> 			Thanks,
> 				Luca
>
> .


Yeah, it looks good.

I can do some experiments with it ,

Do you have some testcases to help me with the test ?


Thanks,

     Cheng Jian.