Re: [PATCH 2/2] locking/qspinlock: Limit # of spins in _Q_PENDING_VAL wait loop

[Will Deacon <will.deacon@arm.com>]

Hi Waiman,

On Mon, Apr 09, 2018 at 02:08:52PM -0400, Waiman Long wrote:
> A locker in the pending code path is doing an infinite number of spins
> when waiting for the _Q_PENDING_VAL to _Q_LOCK_VAL transition. There
> is a concern that lock starvation can happen concurrent lockers are
> able to take the lock in the cmpxchg loop without queuing and pass it
> around amongst themselves.
> 
> To ensure forward progress while still taking advantage of using
> the pending code path without queuing, the code is now modified
> to do a limited number of spins before aborting the effort and
> going to queue itself.
> 
> Ideally, the spinning times should be at least a few times the typical
> cacheline load time from memory which I think can be down to 100ns or
> so for each cacheline load with the newest systems or up to several
> hundreds ns for older systems.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/locking/qspinlock.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 634a49b..35367cc 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -82,6 +82,15 @@
>  #endif
>  
>  /*
> + * The pending bit spinning loop count.
> + * This parameter can be overridden by another architecture specific
> + * constant. Default is 512.
> + */
> +#ifndef _Q_PENDING_LOOP
> +#define _Q_PENDING_LOOP	(1 << 9)
> +#endif

I really dislike heuristics like this because there's never a good number
to choose and it almost certainly varies between systems and workloads
rather than just by architecture. However, I've also not managed to come
up with something better.

If I rewrite your code slightly to look like:

	if (val == _Q_PENDING_VAL) {
		int cnt = _Q_PENDING_LOOP;
		val = atomic_cond_read_relaxed(&lock->val, (VAL != _Q_PENDING_VAL) || !cnt--);
	}

then architectures that implement atomic_cond_read_relaxed as something
more interesting than a spinning loop will probably be happy with
_Q_PENDING_LOOP == 1;

I'll post a v2 tomorrow with that change, and I'll add your stat patch to
the series too so that everything is kept together.

Will