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Subject: Re: [Xen-devel] [PATCH] xen/netfront: raise max number of slots in
 xennet_get_responses()
To:     Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org, netdev@vger.kernel.org
Cc:     davem@davemloft.net
References: <20180611075742.1691-1-jgross@suse.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <e970a52b-d4ad-30c1-65ca-15d7679951da@oracle.com>
Date:   Mon, 11 Jun 2018 14:59:49 -0400
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On 06/11/2018 03:57 AM, Juergen Gross wrote:
> The max number of slots used in xennet_get_responses() is set to
> MAX_SKB_FRAGS + (rx->status <= RX_COPY_THRESHOLD).
>
> In old kernel-xen MAX_SKB_FRAGS was 18, while nowadays it is 17. This
> difference is resulting in frequent messages "too many slots" and a
> reduced network throughput for some workloads (factor 10 below that of
> a kernel-xen based guest).
>
> Replacing MAX_SKB_FRAGS by XEN_NETIF_NR_SLOTS_MIN for calculation of
> the max number of slots to use solves that problem (tests showed no
> more messages "too many slots" and throughput was as high as with the
> kernel-xen based guest system).
>
> Signed-off-by: Juergen Gross <jgross@suse.com>

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

I wonder also whether netfront_tx_slot_available() is meant to be

return (queue->tx.req_prod_pvt - queue->tx.rsp_cons) <
                (NET_TX_RING_SIZE - XEN_NETIF_NR_SLOTS_MIN - 1);

which is the same numeric value but provides a more accurate description
of what is being tested.

-boris


> ---
>  drivers/net/xen-netfront.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 679da1abd73c..ba411005d829 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -790,7 +790,7 @@ static int xennet_get_responses(struct netfront_queue *queue,
>  	RING_IDX cons = queue->rx.rsp_cons;
>  	struct sk_buff *skb = xennet_get_rx_skb(queue, cons);
>  	grant_ref_t ref = xennet_get_rx_ref(queue, cons);
> -	int max = MAX_SKB_FRAGS + (rx->status <= RX_COPY_THRESHOLD);
> +	int max = XEN_NETIF_NR_SLOTS_MIN + (rx->status <= RX_COPY_THRESHOLD);
>  	int slots = 1;
>  	int err = 0;
>  	unsigned long ret;