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SubjectRe: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning
On 04/06/2010 02:15 AM, Darren Hart 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 don't want the context switch path to walk a list whose length is
user controlled.

>> 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.

Yes. O(1) on context switch instead of O(n).

> 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() ?

The lock would need to contain a pointer to the owning task. Could be
set with cmpxchg{8,16}b on x86.

--
error compiling committee.c: too many arguments to function



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