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Date: Mon, 30 May 2016 15:49:11 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Darren Hart <darren@dvhart.com>, Ingo Molnar <mingo@kernel.org>,
        Michael Kerrisk <mtk.manpages@googlemail.com>,
        Davidlohr Bueso <dave@stgolabs.net>, Chris Mason <clm@fb.com>,
        "Carlos O'Donell" <carlos@redhat.com>,
        Torvald Riegel <triegel@redhat.com>,
        Eric Dumazet <edumazet@google.com>
Subject: Re: [patch V2 2/7] futex: Hash private futexes per process
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On Mon, May 30, 2016 at 03:37:48PM +0200, Sebastian Andrzej Siewior wrote:
> Yes, I did not try to discuss the hash collision away. From RT's point
> of view the problems I am aware are of the following scenario:
> Task #1 pinned to CPU1 task #2 pinned to CPU2 but share the same
> bucket. Task #2 got a wakeup and should run but is blocked on the
> bucket lock - otherwise it could run. Usually it would PI-boost task#1
> but task#1 got preempted itself by task and since task#1 prio is lower
> it can't boost its way free towards the lock and so so CPU #2 may
> remain idle for some time.
> 
> The same thing can happen within a Task if you take my story from above
> and replace task with thread. Completely understood.

Right; so I don't see the point of PREALLOCATE_HASH to cater for RT
workloads if it still doesn't guarantee anything, esp. if the failure
case is silent and obscure.