Messages in this thread | | | Date | Mon, 5 Apr 2010 16:29:12 -0700 | From | Chris Wright <> | Subject | Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning |
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* Darren Hart (dvhltc@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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