linux-kernel - Re: [patch V2 2/7] futex: Hash private futexes per process

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Message-ID: <20160530134911.GD3193@twins.programming.kicks-ass.net>
Date:	Mon, 30 May 2016 15:49:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 2/7] futex: Hash private futexes per process

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.