linux-kernel - Re: [RFC V2] mm/vmstat: Add events for HugeTLB migration

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Date:   Wed, 30 Sep 2020 15:32:10 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     linux-mm@...ck.org, Daniel Jordan <daniel.m.jordan@...cle.com>,
        Zi Yan <ziy@...dia.com>, John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC V2] mm/vmstat: Add events for HugeTLB migration

On 09/30/2020 01:16 PM, Oscar Salvador wrote:
> On Wed, Sep 30, 2020 at 11:30:49AM +0530, Anshuman Khandual wrote:
>> -			is_thp = PageTransHuge(page) && !PageHuge(page);
>> -			nr_subpages = thp_nr_pages(page);
>> +			is_thp = false;
>> +			is_hugetlb = false;
>> +			if (PageTransHuge(page)) {
>> +				if (PageHuge(page))
>> +					is_hugetlb = true;
>> +				else
>> +					is_thp = true;
>> +			}
> 
> Since PageHuge only returns true for hugetlb pages, I think the following is
> more simple?
> 
> 	if (PageHuge(page))
> 		is_hugetlb = true;
> 	else if (PageTransHuge(page))
> 		is_thp = true

Right, it would be simple. But as Mike had mentioned before PageHuge()
check is more expensive than PageTransHuge(). This proposal just tries
not to call PageHuge() unless the page first clears PageTransHuge(),
saving some potential CPU cycles on normal pages.

> 
> 
> Besides that, it looks good to me:
> 
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
>