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Re: [RFC] futex: hashbucket as list of futex instead of waiters

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [RFC] futex: hashbucket as list of futex instead of waiters
Date	2017-09-29 10:40 +0200
Message-ID	<uuYZY-3Nm-3@gated-at.bofh.it> (permalink)
References	<uoih4-1pe-9@gated-at.bofh.it> <uuEHW-7Wb-43@gated-at.bofh.it> <uuPMZ-6pH-1@gated-at.bofh.it>
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On Fri, Sep 29, 2017 at 12:39:32AM +0200, Gerd Gerats wrote:
> > 
> > So I suppose the purpose of that plist in futex is to enable waking up
> > the highest prio waiter, but with the advent of SCHED_DEADLINE that no
> > longer works.
> 
> I do not understand, plist is the original data structure. I assume, it works
> together with SCHED_DEADLINE.

plist does not work well with DEADLINE, all deadline tasks will get
lumped together at prio -1; prio as a single integer doesn't really work
for DEADLINE.

We've converted the RT-mutes over to RB-trees a fair while ago; see
commit:

  fb00aca47440 ("rtmutex: Turn the plist into an rb-tree")

> Or is there something on the way not pressent in 4.13.

Thomas held off on converting futex to rb-trees because, as he said
elsewhere in this thread, he's worried about the performance impact on
futexes.

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Re: [RFC] futex: hashbucket as list of futex instead of waiters Peter Zijlstra <peterz@infradead.org> - 2017-09-28 13:00 +0200
  Re: [RFC] futex: hashbucket as list of futex instead of waiters Thomas Gleixner <tglx@linutronix.de> - 2017-09-28 14:20 +0200
  Re: [RFC] futex: hashbucket as list of futex instead of waiters Gerd Gerats <gerd.gerats.lkml@gmx.de> - 2017-09-29 00:50 +0200
    Re: [RFC] futex: hashbucket as list of futex instead of waiters Peter Zijlstra <peterz@infradead.org> - 2017-09-29 10:40 +0200

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