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

[John Hubbard <jhubbard@nvidia.com>]

On 11/18/20 10:17 AM, Dan Williams wrote:
> On Wed, Nov 18, 2020 at 5:51 AM Jan Kara <jack@suse.cz> wrote:
>>
>> 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...
> 
> I don't know what specifically happened in this case, perhaps someone
> from the lkp team can comment? However, the myth / contention that
> "surely someone else would have noticed by now" is why the lkp project
> was launched. Kernels regressed without much complaint and it wasn't
> until much later in the process, around the time enterprise distros
> rebased to new kernels, did end users start filing performance loss
> regression reports. Given -stable kernel releases, 6-7 months is still
> faster than many end user upgrade cycles to new kernel baselines.
> 

I see, thanks for explaining. I'll take a peek, then.

thanks,
-- 
John Hubbard
NVIDIA