netdev - Re: [RFC v2] net: use atomic allocation for order-3 page allocation

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Date:	Thu, 11 Jun 2015 15:53:04 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shaohua Li <shli@...com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, Kernel-team@...com,
	clm@...com, linux-mm@...ck.org, dbavatar@...il.com,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC v2] net: use atomic allocation for order-3 page allocation

On Thu, 2015-06-11 at 15:27 -0700, Shaohua Li wrote:
> We saw excessive direct memory compaction triggered by skb_page_frag_refill.
> This causes performance issues and add latency. Commit 5640f7685831e0
> introduces the order-3 allocation. According to the changelog, the order-3
> allocation isn't a must-have but to improve performance. But direct memory
> compaction has high overhead. The benefit of order-3 allocation can't
> compensate the overhead of direct memory compaction.
> 
> This patch makes the order-3 page allocation atomic. If there is no memory
> pressure and memory isn't fragmented, the alloction will still success, so we
> don't sacrifice the order-3 benefit here. If the atomic allocation fails,
> direct memory compaction will not be triggered, skb_page_frag_refill will
> fallback to order-0 immediately, hence the direct memory compaction overhead is
> avoided. In the allocation failure case, kswapd is waken up and doing
> compaction, so chances are allocation could success next time.
> 
> The mellanox driver does similar thing, if this is accepted, we must fix
> the driver too.
> 
> V2: make the changelog clearer
> 
> Cc: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Shaohua Li <shli@...com>
> ---
>  net/core/sock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 292f422..e9855a4 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1883,7 +1883,7 @@ bool skb_page_frag_refill(unsigned int sz, struct page_frag *pfrag, gfp_t gfp)
>  
>  	pfrag->offset = 0;
>  	if (SKB_FRAG_PAGE_ORDER) {
> -		pfrag->page = alloc_pages(gfp | __GFP_COMP |
> +		pfrag->page = alloc_pages((gfp & ~__GFP_WAIT) | __GFP_COMP |
>  					  __GFP_NOWARN | __GFP_NORETRY,
>  					  SKB_FRAG_PAGE_ORDER);
>  		if (likely(pfrag->page)) {


OK, now what about alloc_skb_with_frags() ?

This should have same problem right ?

Thanks.


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