Re: [External] Re: [v4 4/4] mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO

From: Usama Arif <usama.arif@bytedance.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: linux-mm@kvack.org, muchun.song@linux.dev, rppt@kernel.org,
	linux-kernel@vger.kernel.org, songmuchun@bytedance.com,
	fam.zheng@bytedance.com, liangma@liangbit.com,
	punit.agrawal@bytedance.com
Subject: Re: [External] Re: [v4 4/4] mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO
Date: Wed, 6 Sep 2023 22:27:28 +0100	[thread overview]
Message-ID: <57c8dd7f-d1a0-37c4-1d3b-d6374e92ffa1@bytedance.com> (raw)
In-Reply-To: <20230906181016.GA3612@monkey>

On 06/09/2023 19:10, Mike Kravetz wrote:
> On 09/06/23 12:26, Usama Arif wrote:
>> The new boot flow when it comes to initialization of gigantic pages
>> is as follows:
>> - At boot time, for a gigantic page during __alloc_bootmem_hugepage,
>> the region after the first struct page is marked as noinit.
>> - This results in only the first struct page to be
>> initialized in reserve_bootmem_region. As the tail struct pages are
>> not initialized at this point, there can be a significant saving
>> in boot time if HVO succeeds later on.
>> - Later on in the boot, the head page is prepped and the first
>> HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages
>> are initialized.
>> - HVO is attempted. If it is not successful, then the rest of the
>> tail struct pages are initialized. If it is successful, no more
>> tail struct pages need to be initialized saving significant boot time.
> 
> Code looks reasonable.  Quick question.
> 
> On systems where HVO is disabled, we will still go through this new boot
> flow and init hugetlb tail pages later in boot (gather_bootmem_prealloc).
> Correct?
> If yes, will there be a noticeable change in performance from the current
> flow with HVO disabled?  My concern would be allocating a large number of
> gigantic pages at boot (TB or more).
> 

Thanks for the review.

The patch moves the initialization of struct pages backing hugepage from 
reserve_bootmem_region to a bit later on in the boot to 
gather_bootmem_prealloc. When HVO is disabled, there will be no 
difference in time taken to boot with or without this patch series, as 
262144 struct pages per gigantic page (for x86) are still going to be 
initialized, just in a different place.