Re: [PATCH] rwsem: add rwsem_is_contended

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

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:

DECLARE_RWSEM(foo);
atomic_t is_contended =  ATOMIC_INIT(0);
.
.
.

    /* writing context */
    atomic_inc(&is_contended);
    down_write(&foo);
    do_writing_action();
    up_write(&foo);
    atomic_dec(&is_contended);


    /* reading context */
    down_read(&foo);
    while (!atomic_read(&is_contended))
       do_reading_actions();
    up_read(&foo);


David Daney