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SubjectRe: [mm/gup] 47e29d32af: phoronix-test-suite.npb.FT.A.total_mop_s -45.0% regression
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@suse.com>
SUSE Labs, CR

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