linux-kernel - Re: Next round: revised futex(2) man page for review

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Message-ID: <1438135759.2249.94.camel@stgolabs.net>
Date:	Tue, 28 Jul 2015 19:09:19 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Darren Hart <dvhart@...radead.org>,
	Torvald Riegel <triegel@...hat.com>,
	Carlos O'Donell <carlos@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Jakub Jelinek <jakub@...hat.com>,
	linux-man <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux API <linux-api@...r.kernel.org>,
	Roland McGrath <roland@...k.frob.com>,
	Anton Blanchard <anton@...ba.org>,
	Eric Dumazet <edumazet@...gle.com>,
	bill o gallmeister <bgallmeister@...il.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Daniel Wagner <wagi@...om.org>, Rich Felker <dalias@...c.org>,
	Andy Lutomirski <luto@...capital.net>,
	bert hubert <bert.hubert@...herlabs.nl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Heinrich Schuchardt <xypron.glpk@....de>
Subject: Re: Next round: revised futex(2) man page for review

On Tue, 2015-07-28 at 22:45 +0200, Peter Zijlstra wrote:
> Also, this code seems to use plist, which means it won't do the right
> thing for SCHED_DEADLINE either.

Ick, I don't look forward to seeing nice futex plists converted into
rbtrees. As opposed to, eg. rtmutexes, there are a few caveats:

- Dealing with the top_waiter in rtmutexes is always easy, but in
futexes we need to deal with keys, so caching the leftmost won't work as
nicely.

- This will bloat things like futex_wake, where O(logN) is not suited
for FIFO iteration. And iterating linked lists is, in essence, all that
we really do when calling futex(2).

I have to wonder about the extra overhead added by these points.  I do
understand the dl concern, nonetheless.

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