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?
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