Re: [PATCH] rwsem: add rwsem_is_contended

[Peter Hurley <peter@hurleysoftware.com>]

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, when
a unlocked if (list_empty()) is sufficient.

Regards,
Peter Hurley