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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Oct 19, 2017 at 03:14:07PM +0200, Christoph Hellwig wrote:
> On Thu, Oct 19, 2017 at 07:38:48AM -0400, Brian Foster wrote:
> > On Thu, Oct 19, 2017 at 09:47:05AM +0200, Christoph Hellwig wrote:
> > > Apparently our current rwsem code doesn't like doing the trylock, then
> > > lock for real scheme.  So change our read/write methods to just do the
> > > trylock for the RWF_NOWAIT case.  This fixes a ~25% regression in
> > > AIM7.
> > > 
> > 
> > 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?

--D

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