linux-kernel - Re: [PATCH v3 10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL

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Date:   Mon, 3 Feb 2020 15:16:51 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        <linux-doc@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN)
 reporting

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...


thanks,
-- 
John Hubbard
NVIDIA