Re: [PATCH] rwsem: add rwsem_is_contended

[Josef Bacik <jbacik@fusionio.com>]

On Mon, Sep 16, 2013 at 06:08:42PM -0700, David Daney wrote:
> On 09/16/2013 05:37 PM, Peter Hurley wrote:
> >On 09/16/2013 08:29 PM, David Daney wrote:
> >>On 09/16/2013 05:05 PM, Josef Bacik wrote:
> >>>On Mon, Sep 16, 2013 at 04:05:47PM -0700, Andrew Morton wrote:
> >>>>On Fri, 30 Aug 2013 10:14:01 -0400 Josef Bacik <jbacik@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,
> >>>>>
> >>>>
> >>>>This sounds rather nasty and hacky.  Rather then working around a
> >>>>locking shortcoming in a caller it would be better to fix/enhance the
> >>>>core locking code.  What would such a change need to do?
> >>>>
> >>>>Presently rwsem waiters are fifo-queued, are they not?  So the commit
> >>>>thread will eventually get that lock.  Apparently that's not working
> >>>>adequately for you but I don't fully understand what it is about these
> >>>>dynamics which is causing observable problems.
> >>>>
> >>>
> >>>So the problem is not that its normal lock starvation, it's more our
> >>>particular
> >>>use case that is causing the starvation.  We can have lots of people
> >>>holding
> >>>readers and simply never give them up for long periods of time, which
> >>>is why we
> >>>need this is_contended helper so we know to drop things and let the
> >>>committer
> >>>through.  Thanks,
> >>
> >>You could easily achieve the same thing by putting an "is_contending"
> >>flag in parallel with the rwsem and testing that:
> >
> >Which adds a bunch more bus-locked operations to contended over
> 
> Would that be a problem in this particular case?  Has it been measured?
> 
> >, when
> >a unlocked if (list_empty()) is sufficient.
> 
> I don't object to adding rwsem_is_contended() *if* it is required.  I was
> just pointing out that there may be other options.
> 
> The patch adds a bunch of new semantics to rwsem.  There is a trade off
> between increased complexity of core code, and generalizing subsystem
> specific optimizations that may not be globally useful.
> 
> Is it worth it in this case?  I do not know.
> 

So what you suggested is actually what we did in order to prove that this was
what the problem was.  I'm ok with continuing to do that, I just figured adding
something like rwsem_is_contended() would be nice in case anybody else runs into
the issue in the future, plus it would save me an atomic_t in an already large
structure.  Thanks,

Josef