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

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Date:   Fri, 24 Sep 2021 15:23:16 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Jesper Dangaard Brouer <jbrouer@...hat.com>, <davem@...emloft.net>,
        <kuba@...nel.org>
CC:     <brouer@...hat.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>,
        <hawk@...nel.org>, <ilias.apalodimas@...aro.org>,
        <jonathan.lemon@...il.com>, <alobakin@...me>, <willemb@...gle.com>,
        <cong.wang@...edance.com>, <pabeni@...hat.com>,
        <haokexin@...il.com>, <nogikh@...gle.com>, <elver@...gle.com>,
        <memxor@...il.com>, <edumazet@...gle.com>,
        <alexander.duyck@...il.com>, <dsahern@...il.com>
Subject: Re: [PATCH net-next 2/7] page_pool: support non-split page with
 PP_FLAG_PAGE_FRAG

On 2021/9/23 20:08, Jesper Dangaard Brouer wrote:
> 
> 
> 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@...com>
>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
>> ---
>>   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

Will running these benchmarks to see if any performance overhead noticable here,
thanks for the benchmarks.

> 
>>       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.

The point here is to have consistent handling for both PP_FLAG_PAGE_FRAG
and non-PP_FLAG_PAGE_FRAG case in the following patch.

As the page->_refcount is accessed in "page_ref_count(page) == 1" checking
in __page_pool_put_page(), and page->pp_frag_count is most likely in the
same cache line as the page->_refcount, So I am not expecting a noticable
overhead here.

Anyway, will use the above benchmarks as an example to verify it.


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