netdev - Re: [PATCH] net: convert gro

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Message-ID: <56c49677-8b55-b337-7f5a-c297a11c82b0@gmail.com>
Date:   Wed, 11 Jul 2018 04:32:20 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Li RongQing <lirongqing@...du.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: convert gro_count to bitmask



On 07/11/2018 02:15 AM, Li RongQing wrote:
> gro_hash size is 192 bytes, and uses 3 cache lines, if there is few
> flows, gro_hash may be not fully used, so it is unnecessary to iterate
> all gro_hash in napi_gro_flush(), to occupy unnecessary cacheline.
> 
> convert gro_count to a bitmask, and rename it as gro_bitmask, each bit
> represents a element of gro_hash, only flush a gro_hash element if the
> related bit is set, to speed up napi_gro_flush().
> 
> and update gro_bitmask only if it will be changed, to reduce cache
> update
> 
> Suggested-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>  include/linux/netdevice.h |  2 +-
>  net/core/dev.c            | 35 +++++++++++++++++++++++------------
>  2 files changed, 24 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index b683971e500d..df49b36ef378 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -322,7 +322,7 @@ struct napi_struct {
>  
>  	unsigned long		state;
>  	int			weight;
> -	unsigned int		gro_count;
> +	unsigned long		gro_bitmask;
>  	int			(*poll)(struct napi_struct *, int);
>  #ifdef CONFIG_NETPOLL
>  	int			poll_owner;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index d13cddcac41f..a08dbdd217a6 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5171,9 +5171,11 @@ static void __napi_gro_flush_chain(struct napi_struct *napi, u32 index,
>  			return;
>  		list_del_init(&skb->list);
>  		napi_gro_complete(skb);
> -		napi->gro_count--;
>  		napi->gro_hash[index].count--;
>  	}
> +
> +	if (!napi->gro_hash[index].count)
> +		clear_bit(index, &napi->gro_bitmask);

I suggest you not add an atomic operation here.

Current cpu owns this NAPI after all.

Same remark for the whole patch.

->  __clear_bit(), __set_bit() and similar operators

Ideally you should provide TCP_RR number with busy polling enabled, to eventually catch regressions.

Thanks.