Messages in this thread | | | Date | Mon, 05 Apr 2010 16:15:23 -0700 | From | Darren Hart <> | Subject | Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning |
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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.
> but I don't think it's unreasonable to set a bit in the thread local > storage area. The futex format would then need to be extended to > contain a pointer to this bit.
This appears to be 1 bit per task instead of 1 bit per lock. Also, the value is thread-specific... so how would a potential waiter be able to determine if the owner of a particular lock was running or not with this method? ... maybe I'm missing some core bit about TLS... are you talking about pthread_key_create() and pthread_getspecific() ?
Thanks,
-- Darren Hart IBM Linux Technology Center Real-Time Linux Team
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