Re: [PATCH] rwsem: add rwsem_is_contended

[Peter Hurley]

On 09/03/2013 11:47 AM, Josef Bacik wrote:
> On Sun, Sep 01, 2013 at 01:32:36AM -0700, Michel Lespinasse wrote:
> > Hi Josef,
> >
> > On Fri, Aug 30, 2013 at 7:14 AM, 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,
> > >
> > > Signed-off-by: Josef Bacik <jbacik@xxxxxxxxxxxx>
> >
> > FYI, I once tried to introduce something like this before, but my use
> > case was pretty weak so it was not accepted at the time. I don't think
> > there were any objections to the API itself though, and I think it's
> > potentially a good idea if you use case justifies it.
> >
> > Two comments:
> >
> > - Note that there are two rwsem implementations - if you are going to
> > add functionality to rwsem.h you want to add the same functionality in
> > rwsem-spinlock.h as well.
>
> Sure thing.
>
> > - I would prefer if you could avoid taking the wait_lock in your
> > rwsem.h implementation. In your use case (read lock is known to be
> > held), checking for sem->count < 0 would be sufficient to indicate a
> > writer is queued (or getting onto the queue). In the general case,
> > some architectures have the various values set up so that
> > RWSEM_WAITING_BIAS != RWSEM_ACTIVE_WRITE_BIAS - for these
> > architectures at least, you can check for waiters by looking if the
> > lowest bit of RWSEM_WAITING_BIAS is set in sem->count.
>
> Question about this one, I can't just do
>
> if (sem->count < 0)
>
> since each arch has their own atomic way of looking at count, so I'd have to add
> something to do just a normal read of count for each arch and call that wouldn't
> I?

Reading sem->count is atomic.

For that matter, in your particular use case (which is more heuristic), you
could perform the list_empty() check without acquiring the wait_lock.

Regards,
Peter Hurley



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