Messages in this thread | | | Date | Wed, 30 Oct 2013 09:45:31 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [RFC PATCH] futex: Remove requirement for lock_page in get_futex_key |
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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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