Re: [PATCH 1/2] mm/page_alloc: free order-0 pages through PCP in page_frag_free()

[Jesper Dangaard Brouer]

On Mon,  5 Nov 2018 16:58:19 +0800
Aaron Lu <aaron.lu@xxxxxxxxx> wrote:

> page_frag_free() calls __free_pages_ok() to free the page back to
> Buddy. This is OK for high order page, but for order-0 pages, it
> misses the optimization opportunity of using Per-Cpu-Pages and can
> cause zone lock contention when called frequently.
> 
> PaweÅ Staszewski recently shared his result of 'how Linux kernel
> handles normal traffic'[1] and from perf data, Jesper Dangaard Brouer
> found the lock contention comes from page allocator:
> 
>   mlx5e_poll_tx_cq
>   |
>    --16.34%--napi_consume_skb
>              |
>              |--12.65%--__free_pages_ok
>              |          |
>              |           --11.86%--free_one_page
>              |                     |
>              |                     |--10.10%--queued_spin_lock_slowpath
>              |                     |
>              |                      --0.65%--_raw_spin_lock
>              |
>              |--1.55%--page_frag_free
>              |
>               --1.44%--skb_release_data
> 
> Jesper explained how it happened: mlx5 driver RX-page recycle
> mechanism is not effective in this workload and pages have to go
> through the page allocator. The lock contention happens during
> mlx5 DMA TX completion cycle. And the page allocator cannot keep
> up at these speeds.[2]
> 
> I thought that __free_pages_ok() are mostly freeing high order
> pages and thought this is an lock contention for high order pages
> but Jesper explained in detail that __free_pages_ok() here are
> actually freeing order-0 pages because mlx5 is using order-0 pages
> to satisfy its page pool allocation request.[3]
> 
> The free path as pointed out by Jesper is:
> skb_free_head()
>   -> skb_free_frag()
>     -> skb_free_frag()

Nitpick: you added skb_free_frag() two times, else correct.
(All this stuff gets inlined by the compiler, which makes it hard to
spot with perf report).

>       -> page_frag_free()  
> And the pages being freed on this path are order-0 pages.
> 
> Fix this by doing similar things as in __page_frag_cache_drain() -
> send the being freed page to PCP if it's an order-0 page, or
> directly to Buddy if it is a high order page.
> 
> With this change, PaweÅ hasn't noticed lock contention yet in
> his workload and Jesper has noticed a 7% performance improvement
> using a micro benchmark and lock contention is gone.
> 
> [1]: https://www.spinics.net/lists/netdev/msg531362.html
> [2]: https://www.spinics.net/lists/netdev/msg531421.html
> [3]: https://www.spinics.net/lists/netdev/msg531556.html
> Reported-by: PaweÅ Staszewski <pstaszewski@xxxxxxxxx>
> Analysed-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> ---

It is REALLY great that Aaron spotted this! (based on my analysis).
This have likely been causing scalability issues on real-life network
traffic, but have been hiding behind the driver level recycle tricks
for micro-benchmarking.

Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

>  mm/page_alloc.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ae31839874b8..91a9a6af41a2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4555,8 +4555,14 @@ void page_frag_free(void *addr)
>  {
>  	struct page *page = virt_to_head_page(addr);
>  
> -	if (unlikely(put_page_testzero(page)))
> -		__free_pages_ok(page, compound_order(page));
> +	if (unlikely(put_page_testzero(page))) {
> +		unsigned int order = compound_order(page);
> +
> +		if (order == 0)
> +			free_unref_page(page);
> +		else
> +			__free_pages_ok(page, order);
> +	}
>  }
>  EXPORT_SYMBOL(page_frag_free);
>  

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer