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.
