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

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 6 Apr 2010, Ulrich Drepper wrote:

> On Tue, Apr 6, 2010 at 12:31, Thomas Gleixner <tglx@linutronix.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