Re: [PATCH] rwsem: add rwsem_is_contended

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 30, 2013 at 10:14:01AM -0400, Josef Bacik wrote:
> Btrfs uses an rwsem to control access to its extent tree.  Threads will hold a
> read lock on this rwsem while they scan the extent tree, and if need_resched()
> they will drop the lock and schedule.  The transaction commit needs to take a
> write lock for this rwsem for a very short period to switch out the commit
> roots.  If there are a lot of threads doing this caching operation we can starve
> out the committers which slows everybody out.  To address this we want to add
> this functionality to see if our rwsem has anybody waiting to take a write lock
> so we can drop it and schedule for a bit to allow the commit to continue.


> +/*
> + * check to see if the rwsem we're holding has anybody waiting to acquire it.
> + */
> +int rwsem_is_contended(struct rw_semaphore *sem)
> +{
> +	int ret = 0;
> +	unsigned long flags;
> +
> +	if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
> +		return 1;
> +	if (!list_empty(&sem->wait_list))
> +		ret = 1;
> +	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
> +	return ret;
> +}
> +
> +EXPORT_SYMBOL(rwsem_is_contended);

Modeled after spin_is_contended(), so no problem with that. One thing I
was wondering about is if it wants to be called
rwsem_is_write_contended() or similar, since it explicitly only tests
for pending writers.