Re: [PATCH RFC net-next v2 2/7] net: page_pool: place frag_* fields in one cacheline

[Alexander Lobakin]

From: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
Date: Wed, 26 Jul 2023 11:13:26 +0300

> Apologies for the late reply, I was on vacation and start going
> through my email piles...

No worries. I remember having to grind through hundreds of mails after
each vacation :s :D

> 
> On Tue, 18 Jul 2023 at 16:52, Alexander Lobakin
> <aleksander.lobakin@xxxxxxxxx> wrote:
>>
>> From: Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx>
>> Date: Fri, 14 Jul 2023 20:37:39 +0200
>>
>>>
>>>
>>> On 14/07/2023 19.08, Alexander Lobakin wrote:
>>>> On x86_64, frag_* fields of struct page_pool are scattered across two
>>>> cachelines despite the summary size of 24 bytes. The last field,
>>>> ::frag_users, is pushed out to the next one, sharing it with
>>>> ::alloc_stats.
>>>> All three fields are used in pretty much the same places. There are some
>>>> holes and cold members to move around. Move frag_* one block up, placing
>>>> them right after &page_pool_params perfectly at the beginning of CL2.
>>>> This doesn't do any meaningful to the second block, as those are some
>>>> destroy-path cold structures, and doesn't do anything to ::alloc_stats,
>>>> which still starts at 200-byte offset, 8 bytes after CL3 (still fitting
>>>> into 1 cacheline).
>>>> On my setup, this yields 1-2% of Mpps when using PP frags actively.
>>>> When it comes to 32-bit architectures with 32-byte CL: &page_pool_params
>>>> plus ::pad is 44 bytes, the block taken care of is 16 bytes within one
>>>> CL, so there should be at least no regressions from the actual change.
>>>>
>>>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>>>> ---
>>>>   include/net/page_pool.h | 10 +++++-----
>>>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
>>>> index 829dc1f8ba6b..212d72b5cfec 100644
>>>> --- a/include/net/page_pool.h
>>>> +++ b/include/net/page_pool.h
>>>> @@ -130,16 +130,16 @@ static inline u64
>>>> *page_pool_ethtool_stats_get(u64 *data, void *stats)
>>>>   struct page_pool {
>>>>       struct page_pool_params p;
>>>>   +    long frag_users;
>>>> +    struct page *frag_page;
>>>> +    unsigned int frag_offset;
>>>> +    u32 pages_state_hold_cnt;
>>>
>>> I think this is okay, but I want to highlight that:
>>>  - pages_state_hold_cnt and pages_state_release_cnt
>>> need to be kept on separate cache-lines.
>>
>> They're pretty far away from each other. I moved hold_cnt here as well
>> to keep it stacked with frag_offset and avoid introducing 32-bit holes.
> 
> This is to prevent cache line bouncing and/or false sharing right?
> The change seems fine to me as well but mind adding a comment about
> this when you resend?

Right. Sure, why not.

> 
> Thanks
> /Ilias
>>
>>>
>>>
>>>> +
>>>>       struct delayed_work release_dw;
>>>>       void (*disconnect)(void *);
>>>>       unsigned long defer_start;
>>>>       unsigned long defer_warn;
>>>>   -    u32 pages_state_hold_cnt;
>>>> -    unsigned int frag_offset;
>>>> -    struct page *frag_page;
>>>> -    long frag_users;
>>>> -
>>>>   #ifdef CONFIG_PAGE_POOL_STATS
>>>>       /* these stats are incremented while in softirq context */
>>>>       struct page_pool_alloc_stats alloc_stats;
>>>
>>
>> Thanks,
>> Olek

Thanks,
Olek