Re: [RFC PATCH v2 3/5] futex: Throughput-optimized (TO) futexes

From: Waiman Long
Date: Thu Sep 22 2016 - 16:08:56 EST

On 09/22/2016 11:11 AM, Davidlohr Bueso wrote:
On Thu, 22 Sep 2016, Thomas Gleixner wrote:

On Thu, 22 Sep 2016, Davidlohr Bueso wrote:
On Thu, 22 Sep 2016, Thomas Gleixner wrote:
> Also what's the reason that we can't do probabilistic spinning for
> FUTEX_WAIT and have to add yet another specialized variant of futexes?

Where would this leave the respective FUTEX_WAKE? A nop? Probably have to
differentiate the fact that the queue was empty, but there was a spinning,
instead of straightforward returning 0.

Sorry, but I really can't parse this answer.

Can you folks please communicate with proper and coherent explanations
instead of throwing a few gnawed off bones in my direction?

I actually think that FUTEX_WAIT is the better/nicer approach. But my immediate
question above was how to handle the FUTEX_WAKE counter-part. If we want to
maintain current FIFO ordering for wakeups, now with WAIT spinners this will
create lock stealing scenarios (including if we even guard against starvation).
Or we could reduce the scope of spinners, due to the restrictions, similar to
the top-waiter only being able to spin for rtmutexes. This of course will hurt
the effectiveness of spinning in FUTEX_WAIT in the first place.

Actually, there can be a lot of lock stealing going on with the wait-wake futexes. If the critical section is short enough, many of the lock waiters can be waiting in the hash bucket spinlock queue and not sleeping yet while the futex value changes. As a result, they will exit the futex syscall and back to user space with EAGAIN where one of them may get the lock. So we can't assume that they will get the lock in the FIFO order anyway.

Another immediate thought was situations where we spinner(s) and the wait queue is
empty, the WAKE should also have to acknowledge that situation, as just returning 0
would indicate that there are actually no waiters on the futex.

I would say that adding optimistic spinning to FUTEX_WAIT will be a major change and I don't think it will be less complex than adding a new futex type like the TO futexes while introducing a fair amount of risk of breaking existing applications.