Re: [PATCH 1/6] net: page_pool: Add alloc stats and fast path stat

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 26/01/2022 23.48, Joe Damato wrote:
 > @@ -86,6 +100,7 @@ struct page_pool_params {
 >
 >   struct page_pool {
 >   	struct page_pool_params p;
 > +	struct page_pool_stats ps;
 >
 >   	struct delayed_work release_dw;
The placement of page_pool_stats is problematic, due to cache-line 
placement.

It will be sharing a cache-line with page_pool_params, which is 
read-mostly.  As I say on benchmark email, this is likely the 
performance regression you saw.

I *do* know you have changed location (to percpu) in newer patch 
versions, but I think it is important to point out, that we have to be 
very careful about cache-line placements, and which part of 'struct 
page_pool' is accessed by which part of the code RX or 
DMA-TX-completion "return" code.

--Jesper