Messages in this thread | | | Date | Sat, 31 Aug 2013 16:51:36 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] rwsem: add rwsem_is_contended |
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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.
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