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

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Message-ID: <20151201113036.GW20439@e106622-lin>
Date:	Tue, 1 Dec 2015 11:30:36 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Wanpeng Li <kernellwp@...il.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

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,

- Juri
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