Re: [lkp-robot] [fs/locks] 9d21d181d0: will-it-scale.per_process_ops -14.1% regression

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Jun 01, 2017 at 08:59:21AM -0400, Jeff Layton wrote:
> On Thu, 2017-06-01 at 07:49 -0400, Benjamin Coddington wrote:
> > On 1 Jun 2017, at 7:41, Jeff Layton wrote:
> > 
> > > On Thu, 2017-06-01 at 10:05 +0800, kernel test robot wrote:
> > > > Greeting,
> > > > 
> > > > FYI, we noticed a -14.1% regression of will-it-scale.per_process_ops 
> > > > due to commit:
> > > > 
> > > > 
> > > > commit: 9d21d181d06acab9a8e80eac2ec4eed77b656793 ("fs/locks: Set 
> > > > fl_nspid at file_lock allocation")
> > > > url: 
> > > > https://github.com/0day-ci/linux/commits/Benjamin-Coddington/fs-locks-Alloc-file_lock-where-practical/20170527-050700
> > > > 
> > > > 
> > > 
> > > Ouch, that's a rather nasty performance hit. In hindsight, maybe we
> > > shouldn't move those off the stack after all? Heck, if it's that
> > > significant, maybe we should move the F_SETLK callers to allocate 
> > > these
> > > on the stack as well?
> > 
> > We can do that.  But, I think this is picking up the 
> > locks_mandatory_area()
> > allocation which is now removed.  The attached config has
> > CONFIG_MANDATORY_FILE_LOCKING=y,  so there's allocation on every 
> > read/write.
> > 
> 
> I'm not so sure. That would only be the case if the thing were marked
> for manadatory locking (a really rare thing).
> 
> The test is really simple and I don't think any read/write activity is
> involved:
> 
>     https://github.com/antonblanchard/will-it-scale/blob/master/tests/lock1.c

So it's just F_WRLCK/F_UNLCK in a loop spread across multiple cores?
I'd think real workloads do some work while holding the lock, and a 15%
regression on just the pure lock/unlock loop might not matter?  But best
to be careful, I guess.

--b.

> 
> ...and the 0 day bisected it down to this patch, IIUC:
> 
>     https://github.com/0day-ci/linux/commit/9d21d181d06acab9a8e80eac2ec4eed77b656793
> 
> It seems likely that it's the extra get_pid/put_pid in the allocation
> and free codepath. I expected those to be pretty cheap, but maybe
> they're not?