Re: [mm/gup] 47e29d32af: phoronix-test-suite.npb.FT.A.total_mop_s -45.0% regression

[Jan Kara]

On Mon 16-11-20 19:35:31, John Hubbard wrote:
> 
> On 11/16/20 6:48 PM, kernel test robot wrote:
> > 
> > Greeting,
> > 
> > FYI, we noticed a -45.0% regression of phoronix-test-suite.npb.FT.A.total_mop_s due to commit:
> > 
> 
> That's a huge slowdown...
> 
> > 
> > commit: 47e29d32afba11b13efb51f03154a8cf22fb4360 ("mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages")
> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> 
> ...but that commit happened in April, 2020. Surely if this were a serious
> issue we would have some other indication...is this worth following up
> on?? I'm inclined to ignore it, honestly.

Why this was detected so late is a fair question although it doesn't quite
invalidate the report... The NPB benchmark appears to be a supercomputing
benchmark so concievably it could be heavily using THPs. The question is
why it would be a heavy user of pinning as well but even that is imaginable
considering that MPI is in use etc.

So maybe it is worth trying to reproduce this because heavy THP + pinning
users might be indeed rare and only those would show regressions in THP
pinning performance...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR