netdev - Re: [PATCH net] net: sched: check netif_xmit_frozen_or_stopped() in sch_direct

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Date:   Mon, 28 May 2018 18:30:11 +0000
From:   Song Liu <songliubraving@...com>
To:     John Fastabend <john.fastabend@...il.com>
CC:     Song Liu <liu.song.a23@...il.com>, Alexei Starovoitov <ast@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] net: sched: check netif_xmit_frozen_or_stopped() in
 sch_direct_xmit()



> On May 26, 2018, at 12:43 PM, John Fastabend <john.fastabend@...il.com> wrote:
> 
> On 05/25/2018 12:46 PM, Song Liu wrote:
>> On Fri, May 25, 2018 at 11:11 AM, Song Liu <songliubraving@...com> wrote:
>>> Summary:
>>> 
>>> At the end of sch_direct_xmit(), we are in the else path of
>>> !dev_xmit_complete(ret), which means ret == NETDEV_TX_OK. The following
>>> condition will always fail and netif_xmit_frozen_or_stopped() is not
>>> checked at all.
>>> 
>>>    if (ret && netif_xmit_frozen_or_stopped(txq))
>>>         return false;
>>> 
>>> In this patch, this condition is fixed as:
>>> 
>>>    if (netif_xmit_frozen_or_stopped(txq))
>>>         return false;
>>> 
>>> and further simplifies the code as:
>>> 
>>>    return !netif_xmit_frozen_or_stopped(txq);
>>> 
>>> Fixes: 29b86cdac00a ("net: sched: remove remaining uses for qdisc_qlen in xmit path")
>>> Cc: John Fastabend <john.fastabend@...il.com>
>>> Cc: David S. Miller <davem@...emloft.net>
>>> Signed-off-by: Song Liu <songliubraving@...com>
>>> ---
>>> net/sched/sch_generic.c | 5 +----
>>> 1 file changed, 1 insertion(+), 4 deletions(-)
>>> 
>>> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>>> index 39c144b..8261d48 100644
>>> --- a/net/sched/sch_generic.c
>>> +++ b/net/sched/sch_generic.c
>>> @@ -346,10 +346,7 @@ bool sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
>>>                return false;
>>>        }
>>> 
>>> -       if (ret && netif_xmit_frozen_or_stopped(txq))
>>> -               return false;
>>> -
>>> -       return true;
>>> +       return !netif_xmit_frozen_or_stopped(txq);
>>> }
>>> 
>>> /*
>>> --
>>> 2.9.5
>>> 
>> 
>> Alexei and I discussed about this offline. We would like to share our
>> discussion here to
>> clarify the motivation.
>> 
>> Before 29b86cdac00a, ret in condition "if (ret &&
>> netif_xmit_frozen_or_stopped()" is not
>> the value from dev_hard_start_xmit(), because ret is overwritten by
>> either qdisc_qlen()
>> or dev_requeue_skb(). Therefore, 29b86cdac00a changed the behavior of
>> this condition.
>> 
>> For ret from dev_hard_start_xmit(), I dig into the function and found
>> it is from return value
>> of ndo_start_xmit(). Per netdevice.h, ndo_start_xmit() should only
>> return NETDEV_TX_OK
>> or NETDEV_TX_BUSY. I survey many drivers, and they all follow the rule. The only
>> exception is vlan.
>> 
>> Given ret could only be NETDEV_TX_OK or NETDEV_TX_BUSY (ignore vlan for now),
>> if it fails condition "if (!dev_xmit_complete(ret))", ret must be
>> NETDEV_TX_OK == 0. So
>> netif_xmit_frozen_or_stopped() will always be bypassed.
>> 
>> It is probably OK to ignore netif_xmit_frozen_or_stopped(), and return true from
>> sch_direct_xmit(), as I didn't see that break any functionality. But
>> it is more like "correct
>> by accident" to me. This is the motivation of my original patch.
>> 
>> Alexei pointed out that, the following condition is more like original logic:
>> 
>>      if (qdisc_qlen(q) && netif_xmit_frozen_or_stopped(txq))
>>            return false;
>> 
>> However, I think John would like to remove qdisc_qlen() from the tx
>> path. I didn't see
> 
> Yep qdisc_qlen() is not very friendly for lockless users. At
> some point we will get around to writing a distributed rate
> limiter qdisc and it will be nice to not have to work-around
> qdisc_qlen().
> 
>> any issue without the extra qdisc_qlen() check, so the patch is
>> probably good AS-IS.
>> 
>> Please share your comments and feedback on this.
>> 
> 
> Thanks for the detailed analysis. The above patch looks OK
> to me. Actually I'm debating if we should just drop the check.
> But, there looks to be a case where drivers return NETDEV_TX_OK
> and then stop the queue because it is nearly overrun. By putting
> the check there we stop early instead of doing some extra work
> before realizing the driver ring is full.
> 
> Still this overrun case should be rare so removing the check
> should be OK. Plus as you note its not been running anyways. My
> current recommendation is just remove the check altogether.
> 
> Thanks,
> John 

Thanks John! I will resend a clean up patch to net-next. 

Song