linux-kernel - Re: [tip:core/locking] x86/smp: Move waiting on contended ticket lock out of line

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Date:	Thu, 28 Feb 2013 10:13:45 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	"Low, Jason" <jason.low2@...com>,
	linux-tip-commits@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aquini@...hat.com,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [tip:core/locking] x86/smp: Move waiting on contended ticket
 lock out of line

On 02/27/2013 11:49 PM, Linus Torvalds wrote:
> On Wed, Feb 27, 2013 at 8:06 PM, Davidlohr Bueso <davidlohr.bueso@...com> wrote:
>>
>> The attached file shows how the amount of sys time used by the ipc lock
>> for a 4 and 8 socket box.
>
> I have to say, even with the improvements, that looks pretty
> disgusting. It really makes me wonder if that thing couldn't be done
> better some way. Is it the SysV semaphores that this all ends up
> using, or what?
>
> That said, I think the IPC layer is just about the perfect candidate
> for things like this, because I'm afraid that nobody is ever going to
> fix it.

There's more to it than that. Userspace expects the IPC layer
to provide exclusion and/or serialization. That makes it a
rather poor candidate for parallelism.

Btw, the IPC lock is already fairly fine grained. One ipc
lock is allocated for each set of semaphores allocated through
sys_semget. Looking up those semaphores in the namespace, when
they are used later, is done under RCU.
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