linux-kernel - Re: [RFC PATCH] futex: Remove requirement for lock_page in get_futex

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Message-ID: <alpine.DEB.2.02.1310300929420.19212@ionos.tec.linutronix.de>
Date:	Wed, 30 Oct 2013 09:45:31 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mel Gorman <mgorman@...e.de>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Chris Mason <chris.mason@...ionio.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] futex: Remove requirement for lock_page in
 get_futex_key

On Tue, 29 Oct 2013, Mel Gorman wrote:

> Thomas Gleixner and Peter Zijlstra discussed off-list that real-time users
> currently have a problem with the page lock being contended for unbounded
> periods of time during futex operations. The three of us discussed the
> possibiltity that the page lock is unnecessary in this case because we are
> not concerned with the usual races with reclaim and page cache updates. For
> anonymous pages, the associated futex object is the mm_struct which does
> not require the page lock. For inodes, we should be able to check under
> RCU read lock if the page mapping is still valid to take a reference to
> the inode.  This just leaves one rare race that requires the page lock
> in the slow path. This patch does not completely eliminate the page lock
> but it should reduce contention in the majority of cases.
> 
> Patch boots and futextest did not explode but I did no comparison
> performance tests. Thomas, do you have details of the workload that
> drove you to examine this problem? Alternatively, can you test it and

The scenario is simple. All you need is a PSHARED futex.

Task A 
     get_futex_key()
     lock_page()

---> preemption

Now any other task trying to lock that page will have to wait until
task A gets scheduled back in, which is an unbound time.

It takes quite some time to reproduce, but I'll ask the people who
have that workload to give it a try.

Thanks,

	tglx
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