linux-kernel - Re: [PATCH 1/2] sched: fix RT task-wakeup logic

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Date:	Wed, 23 Apr 2008 16:29:23 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Gregory Haskins" <ghaskins@...ell.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 1/2] sched: fix RT task-wakeup logic

2008/4/23 Gregory Haskins <ghaskins@...ell.com>:
> Dmitry Adamushko pointed out a logic error in task_wake_up_rt() where we
>  will always evaluate to "true".  You can find the thread here:
>
>  http://lkml.org/lkml/2008/4/22/296
>
>  In reality, we only want to try to push tasks away when a wake up request is
>  not going to preempt the current task.  So lets fix it.
>
>  Note: We introduce test_tsk_need_resched() instead of open-coding the flag
>  check so that the merge-conflict with -rt should help remind us that we
>  may need to support NEEDS_RESCHED_DELAYED in the future, too.
>
>  Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>  CC: Dmitry Adamushko <dmitry.adamushko@...il.com>

Acked-by: Dmitry Adamushko <dmitry.adamushko@...il.com>

I have to look at the second patch more thoroughly... but I guess,
we'd be better off opting for something more simple/less complex (it
looks a bit heavy at the first glance).

This patch already fixes an 'obvious' problem in task_wake_up_rt() and
narrows down the scope of the original problem.

For the original scenario (as well as for its variants with SCHED_FIFO
and != priorities) push_rt_tasks() is 'delayed' untill a reschedule
(which is already pending and should take place shortly... provided
'bounded/short' latencies :-)

For T0,T1 having equal prios + SCHED_RR, T1 (which can't be migrated
due to its affinity) will wait till T0's timeslice expires
and for SCHED_FIFO -- till T0 releases a CPU.

The 'optimal' way would be moving T0 immediatelly off this cpu.

-- 
Best regards,
Dmitry Adamushko
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