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

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Date:   Fri, 08 Jan 2021 12:26:58 -0800
From:   Saeed Mahameed <saeed@...nel.org>
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>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH net-next v1 1/2] net: core: count drops from GRO

On Fri, 2021-01-08 at 11:21 -0800, 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 :-) .
> 
> sln
> 

I think there is still some merit in this patchset even with Eric's
removal of GRO_DROP from gro_receive(). As Eric explained, it is still
possible to silently drop for the same reason when drivers
call napi_get_frags or even alloc_skb() apis, many drivers do not
account for such packet drops, and maybe it is the right thing to do to
inline the packet drop accounting into the skb alloc APIs ? the
question is, is it the job of those APIs to update netdev->stats ?