Re: [PATCH] rwsem: add rwsem_is_contended

[David Daney]

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@xxxxxxxxxxxx> 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
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