Re: [PATCH] rwsem: add rwsem_is_contended

[Peter Hurley]

On 09/03/2013 09:18 AM, Josef Bacik wrote:
> On Mon, Sep 02, 2013 at 01:18:08PM -0400, Peter Hurley wrote:
> > On 09/01/2013 04:32 AM, 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.
> >
> > Exactly, I'm concerned about the use case: readers can't starve writers.
> > Of course, lots of existing readers can temporarily prevent a writer from
> > acquiring, but those readers would already have the lock. Any new readers
> > wouldn't be able to prevent a waiting writer from obtaining the lock.
> >
> > Josef,
> > Could you be more explicit, maybe with some detailed numbers about the
> > condition you report?
>
> Sure, this came from a community member
>
> http://article.gmane.org/gmane.comp.file-systems.btrfs/28081
>
> With the old approach we could block between 1-2 seconds waiting for this rwsem,
> and with the new approach where we allow many more of these caching threads we
> were staving out the writer for 80 seconds.
>
> So what happens is these threads will scan our extent tree to put together the
> free space cache, and they'll hold this lock while they are doing the scanning.
> The only way they will drop this lock is if we hit need_resched(), but because
> these threads are going to do quite a bit of IO I imagine we're not ever being
> flagged with need_resched() because we schedule while waiting for IO.  So these
> threads will hold onto this lock for bloody ever without giving it up so the
> committer can take the write lock.  His patch to "fix" the problem was to have
> an atomic that let us know somebody was waiting for a write lock and then we'd
> drop the reader lock and schedule.

Thanks for the additional clarification.

> So really we're just using a rwsem in a really mean way for writers.  I'm open
> to other suggestions but I think this probably the cleanest way.

Is there substantial saved state at the point where the caching thread is
checking need_resched() that precludes dropping and reacquiring the
extent_commit_sem (or before find_next_key())?  Not that it's a cleaner
solution; just want to understand better the situation.

Regards,
Peter Hurley
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/