Re: [PATCH net-next 2/7] page_pool: support non-split page with PP_FLAG_PAGE_FRAG

[Jesper Dangaard Brouer]

On 22/09/2021 11.41, Yunsheng Lin wrote:
> Currently when PP_FLAG_PAGE_FRAG is set, the caller is not
> expected to call page_pool_alloc_pages() directly because of
> the PP_FLAG_PAGE_FRAG checking in __page_pool_put_page().
>
> The patch removes the above checking to enable non-split page
> support when PP_FLAG_PAGE_FRAG is set.
>
> Reviewed-by: Alexander Duyck <alexanderduyck@xxxxxx>
> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> ---
>   net/core/page_pool.c | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index a65bd7972e37..f7e71dcb6a2e 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -315,11 +315,14 @@ struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp)
>   
>   	/* Fast-path: Get a page from cache */
>   	page = __page_pool_get_cached(pool);
> -	if (page)
> -		return page;
>   
>   	/* Slow-path: cache empty, do real allocation */
> -	page = __page_pool_alloc_pages_slow(pool, gfp);
> +	if (!page)
> +		page = __page_pool_alloc_pages_slow(pool, gfp);
> +
> +	if (likely(page))
> +		page_pool_set_frag_count(page, 1);
> +

I really don't like that you add one atomic_long_set operation per page 
alloc call.
This is a fast-path for XDP use-cases, which you are ignoring as you 
drivers doesn't implement XDP.

As I cannot ask you to run XDP benchmarks, I fortunately have some 
page_pool specific microbenchmarks you can run instead.

I will ask you to provide before and after results from running these 
benchmarks [1] and [2].

 [1] 
https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c

 [2] 
https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_cross_cpu.c

How to use these module is documented here[3]:
 [3] 
https://prototype-kernel.readthedocs.io/en/latest/prototype-kernel/build-process.html

>   	return page;
>   }
>   EXPORT_SYMBOL(page_pool_alloc_pages);
> @@ -428,8 +431,7 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
>   		     unsigned int dma_sync_size, bool allow_direct)
>   {
>   	/* It is not the last user for the page frag case */
> -	if (pool->p.flags & PP_FLAG_PAGE_FRAG &&
> -	    page_pool_atomic_sub_frag_count_return(page, 1))
> +	if (page_pool_atomic_sub_frag_count_return(page, 1))
>   		return NULL;

This adds an atomic_long_read, even when PP_FLAG_PAGE_FRAG is not set.

>   
>   	/* This allocator is optimized for the XDP mode that uses