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Date: Mon, 16 Sep 2013 21:22:12 -0400 From: Peter Hurley <peter@...leysoftware.com> To: Josef Bacik <jbacik@...ionio.com> CC: David Daney <ddaney.cavm@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-btrfs@...r.kernel.org, walken@...gle.com, mingo@...e.hu, linux-kernel@...r.kernel.org Subject: Re: [PATCH] rwsem: add rwsem_is_contended On 09/16/2013 09:11 PM, Josef Bacik wrote: > 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@...ionio.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. I saw the original patch you linked to earlier in the discussion, and I agree that for your use case adding a contention test is cleaner and clearer than other options. That said, I think this extension is only useful for readers: writers should be getting their business done and releasing the sem. Also, I think the comment above the function should be clearer that the lock must already be held by the caller; IOW, this is not a trylock replacement. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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