linux-kernel - Re: [PATCH v3] sched/deadline: fix earliest

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Date:	Tue, 1 Dec 2015 20:14:27 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Luca Abeni <luca.abeni@...tn.it>,
	Wanpeng li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] sched/deadline: fix earliest_dl.next logic

2015-12-01 19:30 GMT+08:00 Juri Lelli <juri.lelli@....com>:
> On 30/11/15 10:20, Wanpeng Li wrote:
>> 2015-11-27 20:14 GMT+08:00 Luca Abeni <luca.abeni@...tn.it>:
>> > Hi all,
>> >
>> > I ran some quick tests on this patch (because I was working on something
>> > related), and it seems to me that it triggers a bug. Here are some
>> > information:
>
> [snip]
>
>> >
>> > Here is my understanding of the crash:
>> > - schedule() invokes pick_next_task_dl() which wants to do a context switch
>> > (by selecting
>> >   for execution a new task "p" which is different from "prev")
>> > - pick_next_task_dl() invokes put_prev, which puts the "prev" task in the
>> > pushable tasks
>> >   queue (WARNING! "prev" is still the "current" task in this rq, because the
>> > scheduler is
>> >   still running... I think this is the source of the issue)
>> > - then, pick_next_task_dl() invokes dequeue_pushable_dl_task() on p, to
>> > remove the selected
>> >   task from the pushable tasks queue...
>> > - ...But after your patch dequeue_pushable_dl_task() invokes
>> > pick_next_pushable_dl_task().
>> >   Which sees that the next pushable task is the "current" task (see above).
>> > This happens
>> >   becuase "prev" has already been inserted in the pushable tasks queue, and
>> > can be the
>> >   next pushable task... But "current" has not been updated yet.
>> > - The BUG_ON() at line 1443 of deadline.c is just "BUG_ON(task_current(rq,
>> > p))"
>>
>> Thanks for your report and great analyse, Luca, you are right.
>>
>> >
>> > Summing up, I think pick_next_pushable_dl_task() cannot be called from
>> > dequeue_pushable_dl_task() (at least, not without removing or modifying that
>> > BUG_ON()).
>>
>> Juri, how about remove this BUG_ON() just like rt class?
>>
>
> It seems that we actually check the very same conditions for RT, as we
> do for DL. The difference is that RT doesn't call pick_next_pushable
> _task(). I think we can do the same, just checking and eventually using
> the updated leftmost in dequeue_pushable_dl_task().

Thanks for your suggestion, just send out v4.

Regards,
Wanpeng Li
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