netdev - Re: [PATCH net-next v1 1/2] net: core: count drops from GRO

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Message-ID: <b940a75c-f00b-0dd8-ac33-01278c78210a@mojatatu.com>
Date:   Thu, 14 Jan 2021 08:53:44 -0500
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Shannon Nelson <snelson@...sando.io>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:     netdev@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v1 1/2] net: core: count drops from GRO

On 2021-01-08 2:21 p.m., Shannon Nelson wrote:
> On 1/8/21 10:26 AM, Jesse Brandeburg wrote:
>> Shannon Nelson wrote:
>>
>>> On 1/6/21 1:55 PM, Jesse Brandeburg wrote:
>>>> When drivers call the various receive upcalls to receive an skb
>>>> to the stack, sometimes that stack can drop the packet. The good
>>>> news is that the return code is given to all the drivers of
>>>> NET_RX_DROP or GRO_DROP. The bad news is that no drivers except
>>>> the one "ice" driver that I changed, check the stat and increment
>>> If the stack is dropping the packet, isn't it up to the stack to track
>>> that, perhaps with something that shows up in netstat -s?  We don't
>>> really want to make the driver responsible for any drops that happen
>>> above its head, do we?
>> I totally agree!
>>
>> In patch 2/2 I revert the driver-specific changes I had made in an
>> earlier patch, and this patch *was* my effort to make the stack show the
>> drops.
>>
>> Maybe I wasn't clear. I'm seeing packets disappear during TCP
>> workloads, and this GRO_DROP code was the source of the drops (I see it
>> returning infrequently but regularly)
>>
>> The driver processes the packet but the stack never sees it, and there
>> were no drop counters anywhere tracking it.
>>
> 
> My point is that the patch increments a netdev counter, which to my mind 
> immediately implicates the driver and hardware, rather than the stack. 
> As a driver maintainer, I don't want to be chasing driver packet drop 
> reports that are a stack problem.  I'd rather see a new counter in 
> netstat -s that reflects the stack decision and can better imply what 
> went wrong.  I don't have a good suggestion for a counter name at the 
> moment.
> 
> I guess part of the issue is that this is right on the boundary of 
> driver-stack.  But if we follow Eric's suggestions, maybe the problem 
> magically goes away :-) .
> 

So: How does one know that the stack-upcall dropped a packet because
of GRO issues? Debugging with kprobe or traces doesnt count as an
answer.

cheers,
jamal