netdev - Re: [PATCH v3] net: use bulk free in kfree_skb

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Date:   Mon, 25 Mar 2019 02:27:14 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Felix Fietkau <nbd@....name>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, brouer@...hat.com, fw@...len.de,
        pabeni@...hat.com
Subject: Re: [PATCH v3] net: use bulk free in kfree_skb_list



On 03/25/2019 02:14 AM, Felix Fietkau wrote:
> Since we're freeing multiple skbs, we might as well use bulk free to save a
> few cycles. Use the same conditions for bulk free as in napi_consume_skb.
> 
> Signed-off-by: Felix Fietkau <nbd@....name>
> ---
> v3: reorder checks to prevent skb double unref
> v2: call kmem_cache_free_bulk once the skb array is full instead of
>     falling back to kfree_skb
>  net/core/skbuff.c | 40 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 36 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 2415d9cb9b89..ca0308485669 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -666,12 +666,44 @@ EXPORT_SYMBOL(kfree_skb);
>  
>  void kfree_skb_list(struct sk_buff *segs)
>  {
> -	while (segs) {
> -		struct sk_buff *next = segs->next;
> +	struct sk_buff *next = segs;
> +	void *skbs[16];
> +	int n_skbs = 0;
>  
> -		kfree_skb(segs);
> -		segs = next;
> +	while ((segs = next) != NULL) {
> +		next = segs->next;
> +
> +		if (segs->fclone != SKB_FCLONE_UNAVAILABLE) {
> +			kfree_skb(segs);
> +			continue;
> +		}
> +
> +		if (!skb_unref(segs))
> +			continue;
> +
> +		trace_kfree_skb(segs, __builtin_return_address(0));
> +
> +		/* drop skb->head and call any destructors for packet */
> +		skb_release_all(segs);
> +


> +#ifdef CONFIG_SLUB
> +		/* SLUB writes into objects when freeing */
> +		prefetchw(segs);
> +#endif

This is done too late : 
You should probably either remove this prefetchw()
or do it before reading segs->next at the beginning of the loop.

> +
> +		skbs[n_skbs++] = segs;
> +
> +		if (n_skbs < ARRAY_SIZE(skbs))
> +			continue;
> +
> +		kmem_cache_free_bulk(skbuff_head_cache, n_skbs, skbs);
> +		n_skbs = 0;
>  	}
> +
> +	if (!n_skbs)
> +		return;
> +
> +	kmem_cache_free_bulk(skbuff_head_cache, n_skbs, skbs);
>  }
>  EXPORT_SYMBOL(kfree_skb_list);
>  
>