linux-kernel - Re: [PATCH] sched: Make dl_task_time() use task_rq

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Date:	Tue, 17 Feb 2015 14:32:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] sched: Make dl_task_time() use task_rq_lock()

On Tue, Feb 17, 2015 at 01:31:39PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 17, 2015 at 02:33:58PM +0300, Kirill Tkhai wrote:
> 
> > So, we move task_rq_lock() to sched.h, and dl_task_timer() uses it?
> 
> Yep, like this. I've also modified your earlier patch to dl_task_time()
> back to its original form.
> 
> ---
> Subject: sched: Make dl_task_time() use task_rq_lock()
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Tue Feb 17 13:22:25 CET 2015
> 
> Kirill reported that a dl task can be throttled and dequeued at the
> same time. This happens, when it becomes throttled in schedule(),
> which is called to go to sleep:
> 
> current->state = TASK_INTERRUPTIBLE;
> schedule()
>     deactivate_task()
>         dequeue_task_dl()
>             update_curr_dl()
>                 start_dl_timer()
>             __dequeue_task_dl()
>     prev->on_rq = 0;
> 
> This invalidates the assumption from commit 0f397f2c90ce ("sched/dl:
> Fix race in dl_task_timer()"):
> 
>   "The only reason we don't strictly need ->pi_lock now is because
>    we're guaranteed to have p->state == TASK_RUNNING here and are
>    thus free of ttwu races".
> 
> And therefore we have to use the full task_rq_lock() here.
> 
> This further amends the fact that we forgot to update the rq lock loop
> for TASK_ON_RQ_MIGRATE, from commit cca26e8009d1 ("sched: Teach
> scheduler to understand TASK_ON_RQ_MIGRATING state").
> 
> Reported-by: Kirill Tkhai <ktkhai@...allels.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/sched/core.c     |   76 ------------------------------------------------
>  kernel/sched/deadline.c |   12 +------
>  kernel/sched/sched.h    |   76 ++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 79 insertions(+), 85 deletions(-)

Duh, I put it under CONFIG_SMP, /me changes patch to not do this.
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