Re: [PATCH] rwsem: add rwsem_is_contended

[David Daney <ddaney.cavm@gmail.com>]

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.

<insert quote relating to occam's razor>

David Daney