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 <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, > >>>>> > >>>> > >>>>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 -- 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/
