On Sun, Sep 01, 2013 at 01:32:36AM -0700, Michel Lespinasse wrote: > Hi Josef, > > On Fri, Aug 30, 2013 at 7:14 AM, Josef Bacik <jba...@fusionio.com> 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. > > Thanks, > > > > Signed-off-by: Josef Bacik <jba...@fusionio.com> > > FYI, I once tried to introduce something like this before, but my use > case was pretty weak so it was not accepted at the time. I don't think > there were any objections to the API itself though, and I think it's > potentially a good idea if you use case justifies it. > > Two comments: > > - Note that there are two rwsem implementations - if you are going to > add functionality to rwsem.h you want to add the same functionality in > rwsem-spinlock.h as well. >
Sure thing. > - I would prefer if you could avoid taking the wait_lock in your > rwsem.h implementation. In your use case (read lock is known to be > held), checking for sem->count < 0 would be sufficient to indicate a > writer is queued (or getting onto the queue). In the general case, > some architectures have the various values set up so that > RWSEM_WAITING_BIAS != RWSEM_ACTIVE_WRITE_BIAS - for these > architectures at least, you can check for waiters by looking if the > lowest bit of RWSEM_WAITING_BIAS is set in sem->count. Question about this one, I can't just do if (sem->count < 0) since each arch has their own atomic way of looking at count, so I'd have to add something to do just a normal read of count for each arch and call that wouldn't I? If that's what you want me to do then I'm fine with that (though I'll need a really thorough review), just want to double check before I make a bunch of extra work for myself. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
