netdev - Re: [PATCH][net-next] tun: align write-heavy flow entry members to a cache line

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Date:   Thu, 6 Dec 2018 17:56:13 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Li RongQing <lirongqing@...du.com>, netdev@...r.kernel.org
Subject: Re: [PATCH][net-next] tun: align write-heavy flow entry members to a
 cache line


On 2018/12/6 下午4:08, Li RongQing wrote:
> tun flow entry 'updated' fields are written when receive
> every packet. Thus if a flow is receiving packets from a
> particular flow entry, it'll cause false-sharing with
> all the other who has looked it up, so move it in its own
> cache line
>
> and update 'queue_index' and 'update' field only when
> they are changed to reduce the cache false-sharing.
>
> Signed-off-by: Zhang Yu <zhangyu31@...du.com>
> Signed-off-by: Wang Li <wangli39@...du.com>
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>   drivers/net/tun.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 835c73f42ae7..d0745dc81976 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -201,7 +201,7 @@ struct tun_flow_entry {
>   	u32 rxhash;
>   	u32 rps_rxhash;
>   	int queue_index;
> -	unsigned long updated;
> +	unsigned long updated ____cacheline_aligned_in_smp;
>   };
>   
>   #define TUN_NUM_FLOW_ENTRIES 1024
> @@ -539,8 +539,10 @@ static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
>   	e = tun_flow_find(head, rxhash);
>   	if (likely(e)) {
>   		/* TODO: keep queueing to old queue until it's empty? */
> -		e->queue_index = queue_index;
> -		e->updated = jiffies;
> +		if (e->queue_index != queue_index)
> +			e->queue_index = queue_index;
> +		if (e->updated != jiffies)
> +			e->updated = jiffies;


Acked-by: Jason Wang <jasowang@...hat.com>

Btw, do you have perf numbers for this?

Thanks


>   		sock_rps_record_flow_hash(e->rps_rxhash);
>   	} else {
>   		spin_lock_bh(&tun->lock);