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

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Date:	Mon, 28 Oct 2013 16:36:03 +0800
From:	jianhai luan <jianhai.luan@...cle.com>
To:	David Vrabel <dvrabel@...tab.net>, 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
Subject: Re: [Xen-devel] [PATCH net] xen-netback: use jiffies_64 value to
 calculate credit timeout


On 2013-10-28 16:21, David Vrabel wrote:
> 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.

I agree Annie's suggest. add the below line;

   vif->credit_window_start = now

   vif->credit_timeout.expires = (unsigned long)now;


>>   		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.
I think that we shouldn't add the above line.
>
> David

Jason
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