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From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Davidlohr Bueso <davidlohr@hp.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Jason Low <jason.low2@hp.com>,
	Darren Hart <dvhart@linux.intel.com>,
	Mike Galbraith <efault@gmx.de>, Jeff Mahoney <jeffm@suse.com>,
	Scott Norton <scott.norton@hp.com>, Tom Vaden <tom.vaden@hp.com>,
	Aswin Chandramouleeswaran <aswin@hp.com>,
	Waiman Long <Waiman.Long@hp.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex_wake()
Date: Mon, 2 Dec 2013 13:35:59 +0100	[thread overview]
Message-ID: <20131202123559.GA21756@gmail.com> (raw)
In-Reply-To: <CA+55aFz-v62jWpTqzAhrf5rGS5drbvgG-XHDDN6cdHCLB+kMxg@mail.gmail.com>


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

> On Sun, Dec 1, 2013 at 8:55 AM, Ingo Molnar <mingo@kernel.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

      parent reply	other threads:[~2013-12-02 12:36 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-11-25 20:58 [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex_wake() Thomas Gleixner
2013-11-25 20:58 ` [RFC patch 1/5] futex: Misc cleanups Thomas Gleixner
2013-11-25 20:58 ` [RFC patch 2/5] futex: Document ordering guarantees Thomas Gleixner
2013-11-25 20:58 ` [RFC patch 3/5] futex: Split out unlock from queue_me() Thomas Gleixner
2013-11-25 20:58 ` [RFC patch 4/5] futex: Enqueue waiter before user space check Thomas Gleixner
2013-11-26  0:20   ` Darren Hart
2013-11-25 20:58 ` [RFC patch 5/5] futex: Allow lockless empty check of hash bucket plist Thomas Gleixner
2013-11-26  8:12 ` [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex_wake() Davidlohr Bueso
2013-11-26  8:52   ` Peter Zijlstra
2013-11-26 11:21     ` Ingo Molnar
2013-11-26 11:56       ` Peter Zijlstra
2013-11-26 12:34         ` Thomas Gleixner
2013-11-26 15:38           ` Davidlohr Bueso
2013-11-26 14:49         ` Davidlohr Bueso
2013-11-26 19:25     ` Davidlohr Bueso
2013-11-26 20:51       ` Davidlohr Bueso
2013-11-26 23:56         ` Thomas Gleixner
2013-11-28  7:44           ` Davidlohr Bueso
2013-11-28 11:58             ` Thomas Gleixner
2013-11-28 11:59             ` Peter Zijlstra
2013-11-28 14:23               ` Thomas Gleixner
2013-12-01  4:37               ` Davidlohr Bueso
2013-12-02 11:01                 ` Thomas Gleixner
2013-12-01 12:10               ` Ingo Molnar
2013-12-01 12:56                 ` Peter Zijlstra
2013-12-01 16:55                   ` Ingo Molnar
2013-12-01 18:58                     ` Linus Torvalds
2013-12-01 20:39                       ` Eric Dumazet
2013-12-01 21:46                         ` Linus Torvalds
2013-12-03 17:59                           ` Darren Hart
2013-12-02 12:35                       ` Ingo Molnar [this message]

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