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Subject: Re: [PATCH v3 10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN)
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From:   John Hubbard <jhubbard@nvidia.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
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On 2/3/20 3:16 PM, John Hubbard wrote:
> On 2/3/20 1:34 PM, John Hubbard wrote:
>> On 2/3/20 1:30 PM, Kirill A. Shutemov wrote:
>>> On Mon, Feb 03, 2020 at 01:04:04PM -0800, John Hubbard wrote:
>>>> On 2/3/20 5:53 AM, Kirill A. Shutemov wrote:
>>>>> On Fri, Jan 31, 2020 at 07:40:27PM -0800, John Hubbard wrote:
>>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>>> index c10d0d051c5b..9fe61d15fc0e 100644
>>>>>> --- a/mm/gup.c
>>>>>> +++ b/mm/gup.c
>>>>>> @@ -29,6 +29,19 @@ struct follow_page_context {
>>>>>>  	unsigned int page_mask;
>>>>>>  };
>>>>>>  
>>>>>> +#ifdef CONFIG_DEBUG_VM
>>>>>
>>>>> Why under CONFIG_DEBUG_VM? There's nothing about this in the cover letter.
>>>>>
>>>>
>>>> Early on, gup_benchmark showed a really significant slowdown from using these 
>>>> counters. And I don't doubt that it's still the case.
>>>>
>>>> I'll re-measure and add a short summary and a few numbers to the patch commit
>>>> description, and to the v4 cover letter.
>>>
>>> Looks like you'll show zeros for these counters if debug is off. It can be
>>> confusing to the user. I think these counters should go away if you don't
>>> count them.
>>>
>>
>> OK, that's a good point. (And in fact, the counters==0 situation already led me
>> astray briefly while debugging with Leon R, even. heh.) I'll remove them entirely for
>> the !CONFIG_DEBUG_VM case.
>>
> 
> On second thought, let me do some more careful performance testing. I don't recall
> now if I was just removing every possible perf slowdown item, when I made this decision.
> It could be that the perf is not affected, and I could just leave this feature enabled
> at all times, which would be nicer.
> 
> And after all, these counters were designed for pretty hot-path items. I'll report back
> with results...


Sure enough, any perf effects are hidden in the noise. I'll just remove the CONFIG_DEBUG_VM
checks. Glad you asked about this!



thanks,
-- 
John Hubbard
NVIDIA