linux-kernel - Re: [ANNOUNCE] 3.2.9-rt17

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Date:	Thu, 08 Mar 2012 22:20:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [ANNOUNCE] 3.2.9-rt17

On Thu, 2012-03-08 at 16:08 -0500, Steven Rostedt wrote:
> Hmm, perhaps we need a way to attach a priority to a lock. Maybe we just
> need a way to set a priority of a lock with.. "A task of priority X
> needs the lock, set the owner to at least X while it holds the lock",
> where it doesn't care about the high priority task, it just cares about
> the lock. That is, give locks a priority too (like priority ceiling). On
> doing spin_trylock_rt() (no need for deadlock detection) if it fails,
> gives a lock the priority of the task trying to take it. The lock will
> be given a temporary priority for the duration it is held. The owner of
> the lock will get that priority unless its already higher in priority.
> When the lock is released, both the owner and the lock lose the
> priority.
> 
> Note, spin_trylock_rt() continues to run even on failure.
> 
> Have cpu_chill() do a "sched_yield()" (the good kind, to put the current
> FIFO task behind another FIFO task of the same priority). Then the owner
> of the lock will get to run.
> 
> The sched_yield in cpu_chill() would be needed if the owner of the lock
> is blocked on the lock the high priority task has. After the high
> priority task releases its lock, and calls cpu_chill(), the
> sched_yield() allows the owner of the lock to run if it happens to be
> blocked on the lock the high prio task held. As the cpu_chill() will be
> called after that lock is released. 

Now put the thing on 2 cpus and both tasks can endlessly chase each
other's tail, no? The yield will be useless there.. 


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