Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

[Darren Hart <dvhltc@us.ibm.com>]

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