linux-kernel - Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

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Date:	Wed, 7 Apr 2010 01:16:15 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ulrich Drepper <drepper@...il.com>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Darren Hart <dvhltc@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Peter W. Morreale" <pmorreale@...ell.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Chris Mason <chris.mason@...cle.com>,
	John Cooper <john.cooper@...rd-harmonic.com>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
 spinning

On Tue, 6 Apr 2010, Ulrich Drepper wrote:

> On Tue, Apr 6, 2010 at 12:31, Thomas Gleixner <tglx@...utronix.de> wrote:
> > We need to figure out a more efficient way to
> > do the spinning in the kernel where we have all the necessary
> > information already.
> 
> Really?  The owner information isn't in general available in the
> kernel.  Futex operation doesn't require the value used to be the PID
> (or negative of the PID).  That is a dramatic limitation of the
> usefulness of futexes.

I know that you can do any weird stuff with the futex value, but I
don't see the "dramatic" limitation. Care to elaborate ?

> At userlevel there is access to other fields of the data structure
> which can contain the owner information.
> 
> I would like to see the method using a per-thread pinned page and an
> update of a memory location on scheduling.  For benchmarking at least.

The per thread pinned page would be unconditional, right ?

I agree that benchmarking would be interesting, but OTOH I fear that
we open up a huge can of worms with exposing scheduler details and the
related necessary syscalls like sys_yield_to: User space thread
management/scheduling comes to my mind and I hope we agree that we do
not want to revisit that.

>  I agree that a sys_yield_to() syscall would be at the very least
> useful as well.  But it's useful for other things already.

Useful for what ?

What are the exact semantics of such a syscall ? 

How does that fit into the various scheduling constraints ?

Thanks,

	tglx
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