netdev - Re: [PATCH RFC net-next v2 2/7] net: page_pool: place frag

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Date: Fri, 14 Jul 2023 20:37:39 +0200
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: brouer@...hat.com, Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
 Larysa Zaremba <larysa.zaremba@...el.com>,
 Yunsheng Lin <linyunsheng@...wei.com>,
 Alexander Duyck <alexanderduyck@...com>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next v2 2/7] net: page_pool: place frag_* fields
 in one cacheline



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@...el.com>
> ---
>   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.


> +
>   	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;