Re: [PATCH for-4.14] xfs: fix AIM7 regression

[Christoph Hellwig <hch@lst.de>]

On Thu, Oct 19, 2017 at 03:44:31PM -0700, Darrick J. Wong wrote:
> > > The code looks fine, but this seems really strange. If the trylock
> > > fails, then wouldn't the blocking lock have slept anyways if done
> > > initially? Is there any more background info available on this, or
> > > perhaps a theory on why there is such a significant regression..?
> > 
> > No, unfortunately I don't have a theory, but I agree it is odd
> > behavior in the rwsem code.
> 
> <shrug> I want to know a little more about why there's a performance hit
> in the down_read_trylock -> down_read case.  Are we getting penalized
> for that?  Is it some weird interaction with lockdep?

I don't think the test bot did run with lockdep.  But feel free to take
a look at the mail thread titled

[lkp-robot] [fs]  91f9943e1c:  aim7.jobs-per-min -26.6% regression

on lkml.  Note that synthetic benchmarks on XFS always saw weird
effects from rwsem details.  I remember that a few years ago I had
to back to the mainline patch to move the rwsem fastpath out of line
because thay caused a major performance regressions on CIFS file
serving benchmarks on a very low end ARM NAS box.