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

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Date:   Thu, 30 Sep 2021 15:28:39 +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: [Linuxarm] Re: [PATCH net-next 2/7] page_pool: support non-split
 page with PP_FLAG_PAGE_FRAG

On 2021/9/24 15:23, Yunsheng Lin wrote:
> 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.

You are right, there is notiable overhead for bench_page_pool_cross_cpu test
case above, possibly due to the cache bouncing caused by page_pool_set_frag_count().

As memntioned by Ilias, mlx5 use page pool and also do the recycling internally,
so handling the page frag tracking consistently for both PP_FLAG_PAGE_FRAG and
non-PP_FLAG_PAGE_FRAG will break the mlx5 driver.

So I will drop this patch for now.

> 
>>
>>>       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
>>>
>>
>> .
>>
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