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SubjectRe: [PATCH 4/4] futex: Add FUTEX_LOCK with optional adaptive spinning
Thomas Gleixner wrote:
> On Wed, 5 May 2010, Darren Hart wrote:
>
>> Add a non-pi TID value based futex locking mechanism. This enables the
>> use of adaptive spinning which was problematic with the basic FUTEX_WAIT
>> operation.
>
> You still do way too much work in that spin code with way too much
> code lines.
>
> Can you try the following (completely uncompiled/untested) patch ?
>
> Thanks,
>
> tglx
> Subject: futex-simplify.patch
> From: Thomas Gleixner <tglx@linutronix.de>
> Date: Fri, 07 May 2010 17:56:38 +0200
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> kernel/futex.c | 42 ++++++++++++------------------------
> kernel/sched.c | 66 ---------------------------------------------------------
> 2 files changed, 14 insertions(+), 94 deletions(-)
>


> + if (to && !to->task) {
> + ret = -ETIMEOUT;
> break;

I had hoped to be able to do it like this too. Unfortunately, we can't
arm the timer until after we set TASK_INTERRUPTIBLE, and we don't do
that until after we give up on spinning.


> -
> - if (timeout) {
> - now = ktime_get();
> - if (timeout->tv64 < now.tv64)
> - break;
> }

This bit clearly needs work. Either via some interpolated time
calculation using something like jiffies, or via another timer that sets
a bit we can check in here.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team


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