linux-kernel - Re: [RFC][PATCH 3/3] staging: ion: Avoid using rt

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Message-ID: <CAMbhsRQOWKWBjQJWndr57Cz_Xf0EeU8jNMJOyiOEBHFRf=Tt=Q@mail.gmail.com>
Date:	Mon, 16 Dec 2013 16:17:57 -0800
From:	Colin Cross <ccross@...roid.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [RFC][PATCH 3/3] staging: ion: Avoid using rt_mutexes directly.

On Mon, Dec 16, 2013 at 1:32 PM, John Stultz <john.stultz@...aro.org> wrote:
> RT_MUTEXES can be configured out of the kernel, causing compile
> problems with ION.
>
> Since there is no documentation as to why directly using rt_mutexes
> is necessary (and very few drivers directly use rt_mutexes), simply
> convert the ion_heap lock to a normal mutex.

rt_mutexes were added with the deferred freeing feature.  Heaps need
to return zeroed memory to userspace, but zeroing the memory on every
allocation was causing performance issues.  We added a SCHED_IDLE
thread to zero memory in the background after freeing, but locking the
heap from the SCHED_IDLE thread might block a high priority allocation
thread for a long time.

The lock is only used to protect the heap's free_list and
free_list_size members, and is not held for any long or sleeping
operations.  Converting to a spinlock should prevent priority
inversion without using the rt_mutex.  I'd also rename it to free_lock
to so it doesn't get used as a general heap lock.
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