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Date: Mon, 28 Oct 2013 14:28:28 +0800
From: jianhai luan <jianhai.luan@...cle.com>
To: annie li <annie.li@...cle.com>, Wei Liu <wei.liu2@...rix.com>
CC: Ian Campbell <ian.campbell@...rix.com>, netdev@...r.kernel.org,
xen-devel@...ts.xen.org, 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 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 <jbeulich@...e.com>
>>> Suggested-by: David Vrabel <david.vrabel@...rix.com>
>>> 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 */
>>> + 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
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