Messages in this thread | | | Date | Thu, 08 Apr 2010 22:41:32 -0700 | From | Darren Hart <> | Subject | Re: [PATCH 4/6] futex: Add FUTEX_LOCK with optional adaptive spinning |
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Peter W. Morreale wrote: > On Wed, 2010-04-07 at 20:25 -0700, Darren Hart wrote: >> Thomas Gleixner wrote: >>> On Wed, 7 Apr 2010, Darren Hart wrote: >>>> Thomas Gleixner wrote:
>>>>> if ((curval & FUTEX_TID_MASK) != ownertid) { >>>>> ownertid = curval & FUTEX_TID_MASK; >>>>> owner = update_owner(ownertid); >>>>> } >>>> Hrm... at this point the owner has changed... so we should break and go >>>> to sleep, not update the owner and start spinning again. The >>>> futex_spin_on_owner() will detect this and abort, so I'm not seeing the >>>> purpose of the above if() block. >>> Why ? If the owner has changed and the new owner is running on another >>> cpu then why not spin further ? >> That's an interesting question, and I'm not sure what the right answer >> is. The current approach of the adaptive spinning in the kernel is to >> spin until the owner changes or deschedules, then stop and block. The >> idea is that if you didn't get the lock before the owner changed, you >> aren't going to get it in a very short period of time (you have at least >> an entire critical section to wait through plus whatever time you've >> already spent spinning). However, blocking just so another task can spin >> doesn't really make sense either, and makes the lock less fair than it >> could otherwise be. > > Not only less fair, but potentially could cause starvation, no? Perhaps > you could see this if you changed your model to allow all contended > tasks to spin instead of just one.
Agreed, and V5 (just posted) does just that.
> > If a spinning task blocks because of an owner change, and a new task > enters and starts spinning directly after the owner change, at what > point does the original task get woken up?
At the time of unlock the owner will have to call FUTEX_WAKE. This task will wake and attempt to acquire the lock - it will lose races with aclready running contenders. Lock stealing, adaptive spinning, etc are all going to lead to less fair locks in exchange for throughput.
> Its likely that the new > spinner will get the resource next, no? Rinse/repeat with another task > and the original spinner is starved. > > (Or am I missing something? My understanding was that unfairness was > built-in to this algo... If so, then the above is a possibility, right?)
Yes it is. These locks are typically used in situations where it is more important that some work gets completed than _which_ work gets completed.
Thanks,
-- Darren Hart IBM Linux Technology Center Real-Time Linux Team
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