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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2023/11/27 22:08, Alexander Lobakin wrote:
> From: Yunsheng Lin <linyunsheng@huawei.com>
> 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@intel.com>
>>> ---
>>>  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.

I am still a little lost here, A comment explaining why using '4' in the
above would be really helpful here.