linux-kernel - Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex

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Message-ID: <20131202123559.GA21756@gmail.com>
Date:	Mon, 2 Dec 2013 13:35:59 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	Scott Norton <scott.norton@...com>,
	Tom Vaden <tom.vaden@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket
 plist in futex_wake()


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Sun, Dec 1, 2013 at 8:55 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > Keeping the page invariant would bring significant performance 
> > advantages to hashing.
> 
> Or not. Rather, it would make things much worse. The virtual address 
> is much simpler and better to avoid needing any page table lookup 
> etc crap. The key is just the mm and the virtual address, and no 
> silly page table walks etc necessary.

In theory the CPU could give us the phyisical page address, as the TLB 
is already there: for most futex ops when the kernel runs we just 
attempted access of the virtual address on the user-space side, so the 
hardware already did the hard work of looking up everything in the 
page tables and has it cached most likely.

But yeah, you are right :-/

Thanks,

	ngo
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