linux-kernel - Re: [PATCH] mm/hugetlb: avoid unnecessary hugetlb_acct

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Date:   Fri, 15 Jan 2021 10:04:19 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/hugetlb: avoid unnecessary hugetlb_acct_memory() call

Hi:

On 2021/1/15 3:16, Mike Kravetz wrote:
> On 1/14/21 4:32 AM, David Hildenbrand wrote:
>> On 14.01.21 12:31, Miaohe Lin wrote:
>>> When gbl_reserve is 0, hugetlb_acct_memory() will do nothing except holding
>>> and releasing hugetlb_lock.
>>
>> So, what's the deal then? Adding more code?
>>
>> If this is a performance improvement, we should spell it out. Otherwise
>> I don't see a real benefit of this patch.
>>
> 
> Thanks for finding/noticing this.
> 
> As David points out, the commit message should state that this is a
> performance improvement.  Mention that such a change avoids an unnecessary
> hugetlb_lock lock/unlock cycle.  You can also mention that this unnecessary
> lock cycle is happening on 'most' hugetlb munmap operations.
> 

My bad. I should spell this out explicitly. Many thanks for both of you.

>>>
>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>> ---
>>>  mm/hugetlb.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 737b2dce19e6..fe2da9ad6233 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -5241,7 +5241,8 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end,
>>>  	 * reservations to be released may be adjusted.
>>>  	 */
>>>  	gbl_reserve = hugepage_subpool_put_pages(spool, (chg - freed));
>>> -	hugetlb_acct_memory(h, -gbl_reserve);
>>> +	if (gbl_reserve)
>>> +		hugetlb_acct_memory(h, -gbl_reserve);
> 
> It is true that gbl_reserve is likely to be 0 in this code path.  However,
> there are other code paths where hugetlb_acct_memory is called with a delta
> value of 0 as well.  I would rather see a simple check at the beginning of
> hugetlb_acct_memory like.
> 
> 	if (!delta)
> 		return 0;
> 

Sounds good. Will do it in v2. Many thanks again.