Subject: Re: [Xen-devel] [PATCH net] xen-netback: use jiffies_64 value to calculate credit timeout

From: jianhai luan <[email-masked]>
Date: Mon, 28 Oct 2013 14:28:28 +0800

On 2013-10-28 10:58, annie li wrote:
>
> On 2013-10-28 10:36, annie li wrote:
>>
>> On 2013-10-27 19:11, 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.
>>>
>>> Suggested-by: Jan Beulich <[email-masked]>
>>> Suggested-by: David Vrabel <[email-masked]>
>>> Signed-off-by: Wei Liu <[email-masked]>
>>> Cc: Ian Campbell <[email-masked]>
>>> Cc: Jason Luan <[email-masked]>
>>> ---
>>>   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 */
>>> +    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);
>>
>> You simply replace "credit_timeout.expires" with 
>> "vif->credit_window_start" here, and never update 
>> "vif->credit_window_start" in following code.
>>
>>>         /* 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;
>>
>> updates credit_window_start as following,
>> vif->credit_window_start = (unsigned long)now;
>
> both credit_window_start and credit_timeout.expires need to be updated 
> here,
>
> vif->credit_window_start = (unsigned long)now;
> vif->credit_timeout.expires = (unsigned long)now;

Annie should be correct.
It is good to me but i afraid that get_jiffies_64() and 
time_after_eq64() will bring some  performance's load when transmit is 
busy in 32bit.  I will do some test about the patch.

No matter what, i prefer own patch.

Thanks,
Jason
>
>>
>>>           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);
>>
>> vif->credit_timeout.expires is unsigned long, and this still causes 
>> original issue on 32bit system, which works well on 64bit. Or 
>> rewriting code to avoid uses of vif->credit_timeout.expires, but it 
>> is complex.
> My understanding here is wrong, please ignore this.
>
> Thanks
> Annie