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

From: Alan Cox
Date: Tue Apr 06 2010 - 12:42:04 EST

> Do you feel some of these situations would also benefit from some kernel
> assistance to stop spinning when the owner schedules out? Or are you
> saying that there are situations where pure userspace spinlocks will
> always be the best option?

There are cases its the best option - you are assuming for example that
the owner can get scheduled out. Eg nailing one thread per CPU in some
specialist high performance situations means they can't.

> If the latter, I'd think that they would also be situations where
> sched_yield() is not used as part of the spin loop. If so, then these
> are not our target situations for FUTEX_LOCK_ADAPTIVE, which hopes to
> provide a better informed mechanism for making spin or sleep decisions.
> If sleeping isn't part of the locking construct implementation, then
> FUTEX_LOCK_ADAPTIVE doesn't have much to offer.

I am unsure about the approach. As Avi says knowing that the lock owner is
scheduled out allows for far better behaviour. It doesn't need complex
per lock stuff or per lock notifier entries on pre-empt either.

A given task is either pre-empted or not and in the normal case of things
you need this within a process so you've got shared pages anyway. So you
only need one instance of the 'is thread X pre-empted' bit somewhere in a
non swappable page.

That gives you something along the lines of

runaddr = find_run_flag(lock);
do {
while(*runaddr == RUNNING) {
if (trylock(lock))
return WHOOPEE;
cpu relax
yield (_on(thread));
} while(*runaddr != DEAD);

which unlike blindly spinning can avoid the worst of any hit on the CPU
power and would be a bit more guided ?

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at