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From: Chris Wright on 5 Apr 2010 19:40
* Darren Hart (dvhltc(a)us.ibm.com) wrote:
> Avi Kivity wrote:
>
>>>> An interesting (but perhaps difficult to achieve) optimization
>>>> would be to spin in userspace.
>>>
>>> I couldn't think of a lightweight way to determine when the owner has
>>> been scheduled out in userspace. Kernel assistance is required. You
>>> could do this on the schedule() side of things, but I figured I'd get
>>> some strong pushback if I tried to add a hook into descheduling that
>>> flipped a bit in the futex value stating the owner was about to
>>> deschedule(). Still, that might be something to explore.
>>
>> In the futex value it's hopeless (since a thread can hold many locks),
>
> It can, but there is a futex value per lock. If the task_struct had a
> list of held futex locks (as it does for pi futex locks) the
> deschedule() path could walk that and mark the FUTEX_OWNER_SLEEPING bit.

You also have a notification scheme (preempt_notifiers will fire on
schedule out). However, you'd have to register the notifiers from a
non-current context (i.e. on slow path acquire reaching out to lock owner
and registering notifier on lock owner's behalf, which would kind of
defeat the point AFAICT).

thanks,
-chris
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