Messages in this thread | | | Date | Wed, 7 Apr 2010 01:16:15 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning |
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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
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