Re: [PATCH net-next v5 01/14] page_pool: make sure frag API fields don't span between cachelines

[Alexander Lobakin]

From: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
Date: Sat, 25 Nov 2023 20:29:22 +0800

> On 2023/11/24 23:47, Alexander Lobakin wrote:
>> After commit 5027ec19f104 ("net: page_pool: split the page_pool_params
>> into fast and slow") that made &page_pool contain only "hot" params at
>> the start, cacheline boundary chops frag API fields group in the middle
>> again.
>> To not bother with this each time fast params get expanded or shrunk,
>> let's just align them to `4 * sizeof(long)`, the closest upper pow-2 to
>> their actual size (2 longs + 2 ints). This ensures 16-byte alignment for
>> the 32-bit architectures and 32-byte alignment for the 64-bit ones,
>> excluding unnecessary false-sharing.
>>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>> ---
>>  include/net/page_pool/types.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/net/page_pool/types.h b/include/net/page_pool/types.h
>> index e1bb92c192de..989d07b831fc 100644
>> --- a/include/net/page_pool/types.h
>> +++ b/include/net/page_pool/types.h
>> @@ -127,7 +127,7 @@ struct page_pool {
>>  
>>  	bool has_init_callback;
> 
> It seems odd to have only a slow field between tow fast
> field group, isn't it better to move it to the end of
> page_pool or where is more appropriate?

1. There will be more in the subsequent patches.
2. ::has_init_callback happens each new page allocation, it's not slow.
   Jakub did put it here for purpose.

> 
>>  
>> -	long frag_users;
>> +	long frag_users __aligned(4 * sizeof(long));
> 
> If we need that, why not just use '____cacheline_aligned_in_smp'?

It can be an overkill. We don't need a full cacheline, but only these
fields to stay within one, no matter whether they are in the beginning
of it or at the end.

> 
>>  	struct page *frag_page;
>>  	unsigned int frag_offset;
>>  	u32 pages_state_hold_cnt;
>>

Thanks,
Olek