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

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Message-ID: <b7587d72-fb5b-4e0f-4fa0-d63e035e521c@oracle.com>
Date:   Thu, 14 Jan 2021 11:16:05 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: avoid unnecessary hugetlb_acct_memory() call

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.

>>
>> 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;

-- 
Mike Kravetz

>>  
>>  	return 0;
>>  }
>>
> 
>