netdev - Re: [Xen-devel] [PATCH net] xen-netback: use jiffies

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Message-ID: <526E1E97.7090804@cantab.net>
Date:	Mon, 28 Oct 2013 08:21:43 +0000
From:	David Vrabel <dvrabel@...tab.net>
To:	Wei Liu <wei.liu2@...rix.com>, xen-devel@...ts.xen.org,
	netdev@...r.kernel.org
CC:	Ian Campbell <ian.campbell@...rix.com>, annie.li@...cle.com,
	david.vrabel@...rix.com, jbeulich@...e.com,
	Jason Luan <jianhai.luan@...cle.com>
Subject: Re: [Xen-devel] [PATCH net] xen-netback: use jiffies_64 value to
 calculate credit timeout

On 27/10/2013 11:11, Wei Liu wrote:
> time_after_eq() only works if the delta is < MAX_ULONG/2.
> 
> For a 32bit Dom0, if netfront sends packets at a very low rate, the time
> between subsequent calls to tx_credit_exceeded() may exceed MAX_ULONG/2
> and the test for timer_after_eq() will be incorrect. Credit will not be
> replenished and the guest may become unable to send packets (e.g., if
> prior to the long gap, all credit was exhausted).
> 
> Use jiffies_64 variant to mitigate this problem for 32bit Dom0.

Thanks.

> Suggested-by: Jan Beulich <jbeulich@...e.com>
> Suggested-by: David Vrabel <david.vrabel@...rix.com>

Jan's suggestion; I just agreed with him.

> Signed-off-by: Wei Liu <wei.liu2@...rix.com>
> Cc: Ian Campbell <ian.campbell@...rix.com>
> Cc: Jason Luan <jianhai.luan@...cle.com>
> ---
>  drivers/net/xen-netback/common.h    |    1 +
>  drivers/net/xen-netback/interface.c |    4 ++--
>  drivers/net/xen-netback/netback.c   |   13 ++++++-------
>  3 files changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
> index 5715318..400fea1 100644
> --- a/drivers/net/xen-netback/common.h
> +++ b/drivers/net/xen-netback/common.h
> @@ -163,6 +163,7 @@ struct xenvif {
>  	unsigned long   credit_usec;
>  	unsigned long   remaining_credit;
>  	struct timer_list credit_timeout;
> +	u64 credit_window_start;
>  
>  	/* Statistics */
>  	unsigned long rx_gso_checksum_fixup;
> diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
> index 01bb854..8c45b63 100644
> --- a/drivers/net/xen-netback/interface.c
> +++ b/drivers/net/xen-netback/interface.c
> @@ -312,8 +312,8 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
>  	vif->credit_bytes = vif->remaining_credit = ~0UL;
>  	vif->credit_usec  = 0UL;
>  	init_timer(&vif->credit_timeout);
> -	/* Initialize 'expires' now: it's used to track the credit window. */
> -	vif->credit_timeout.expires = jiffies;
> +	/* credit window is tracked in credit_window_start */

You could drop this comment as the field name makes this obvious.

> +	vif->credit_window_start = get_jiffies_64();
>  
>  	dev->netdev_ops	= &xenvif_netdev_ops;
>  	dev->hw_features = NETIF_F_SG | NETIF_F_IP_CSUM | NETIF_F_TSO;
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index f3e591c..1bc0688 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1185,18 +1185,17 @@ out:
>  
>  static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
>  {
> -	unsigned long now = jiffies;
> -	unsigned long next_credit =
> -		vif->credit_timeout.expires +
> -		msecs_to_jiffies(vif->credit_usec / 1000);
> +	u64 now = get_jiffies_64();
> +	u64 next_credit = vif->credit_window_start +
> +		(u64)msecs_to_jiffies(vif->credit_usec / 1000);
>  
>  	/* Timer could already be pending in rare cases. */
>  	if (timer_pending(&vif->credit_timeout))
>  		return true;
>  
>  	/* Passed the point where we can replenish credit? */
> -	if (time_after_eq(now, next_credit)) {
> -		vif->credit_timeout.expires = now;
> +	if (time_after_eq64(now, next_credit)) {
> +		vif->credit_timeout.expires = (unsigned long)now;

Need to set vif->credit_window_start = now here instead of .expires.

>  		tx_add_credit(vif);
>  	}
>  
> @@ -1207,7 +1206,7 @@ static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
>  		vif->credit_timeout.function =
>  			tx_credit_callback;
>  		mod_timer(&vif->credit_timeout,
> -			  next_credit);
> +			  (unsigned long)next_credit);

Need to set vif->credit_window_start = next_credit here as well.

David
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