Date:   Fri, 12 Jul 2019 20:40:11 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Ido Schimmel <idosch@...sch.org>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        jiri@...lanox.com, mlxsw@...lanox.com, dsahern@...il.com,
        roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
        andy@...yhouse.net, pablo@...filter.org,
        jakub.kicinski@...ronome.com, pieter.jansenvanvuuren@...ronome.com,
        andrew@...n.ch, f.fainelli@...il.com, vivien.didelot@...il.com,
        idosch@...lanox.com
Subject: Re: [PATCH net-next 00/11] Add drop monitor for offloaded data paths

On Fri, Jul 12, 2019 at 02:33:29PM +0200, Toke Høiland-Jørgensen wrote:
> Neil Horman <nhorman@...driver.com> writes:
> 
> > On Fri, Jul 12, 2019 at 11:27:55AM +0200, Toke Høiland-Jørgensen wrote:
> >> Neil Horman <nhorman@...driver.com> writes:
> >> 
> >> > On Thu, Jul 11, 2019 at 03:39:09PM +0300, Ido Schimmel wrote:
> >> >> On Sun, Jul 07, 2019 at 12:45:41PM -0700, David Miller wrote:
> >> >> > From: Ido Schimmel <idosch@...sch.org>
> >> >> > Date: Sun,  7 Jul 2019 10:58:17 +0300
> >> >> > 
> >> >> > > Users have several ways to debug the kernel and understand why a packet
> >> >> > > was dropped. For example, using "drop monitor" and "perf". Both
> >> >> > > utilities trace kfree_skb(), which is the function called when a packet
> >> >> > > is freed as part of a failure. The information provided by these tools
> >> >> > > is invaluable when trying to understand the cause of a packet loss.
> >> >> > > 
> >> >> > > In recent years, large portions of the kernel data path were offloaded
> >> >> > > to capable devices. Today, it is possible to perform L2 and L3
> >> >> > > forwarding in hardware, as well as tunneling (IP-in-IP and VXLAN).
> >> >> > > Different TC classifiers and actions are also offloaded to capable
> >> >> > > devices, at both ingress and egress.
> >> >> > > 
> >> >> > > However, when the data path is offloaded it is not possible to achieve
> >> >> > > the same level of introspection as tools such "perf" and "drop monitor"
> >> >> > > become irrelevant.
> >> >> > > 
> >> >> > > This patchset aims to solve this by allowing users to monitor packets
> >> >> > > that the underlying device decided to drop along with relevant metadata
> >> >> > > such as the drop reason and ingress port.
> >> >> > 
> >> >> > We are now going to have 5 or so ways to capture packets passing through
> >> >> > the system, this is nonsense.
> >> >> > 
> >> >> > AF_PACKET, kfree_skb drop monitor, perf, XDP perf events, and now this
> >> >> > devlink thing.
> >> >> > 
> >> >> > This is insanity, too many ways to do the same thing and therefore the
> >> >> > worst possible user experience.
> >> >> > 
> >> >> > Pick _ONE_ method to trap packets and forward normal kfree_skb events,
> >> >> > XDP perf events, and these taps there too.
> >> >> > 
> >> >> > I mean really, think about it from the average user's perspective.  To
> >> >> > see all drops/pkts I have to attach a kfree_skb tracepoint, and not just
> >> >> > listen on devlink but configure a special tap thing beforehand and then
> >> >> > if someone is using XDP I gotta setup another perf event buffer capture
> >> >> > thing too.
> >> >> 
> >> >> Dave,
> >> >> 
> >> >> Before I start working on v2, I would like to get your feedback on the
> >> >> high level plan. Also adding Neil who is the maintainer of drop_monitor
> >> >> (and counterpart DropWatch tool [1]).
> >> >> 
> >> >> IIUC, the problem you point out is that users need to use different
> >> >> tools to monitor packet drops based on where these drops occur
> >> >> (SW/HW/XDP).
> >> >> 
> >> >> Therefore, my plan is to extend the existing drop_monitor netlink
> >> >> channel to also cover HW drops. I will add a new message type and a new
> >> >> multicast group for HW drops and encode in the message what is currently
> >> >> encoded in the devlink events.
> >> >> 
> >> > A few things here:
> >> > IIRC we don't announce individual hardware drops, drivers record them in
> >> > internal structures, and they are retrieved on demand via ethtool calls, so you
> >> > will either need to include some polling (probably not a very performant idea),
> >> > or some sort of flagging mechanism to indicate that on the next message sent to
> >> > user space you should go retrieve hw stats from a given interface.  I certainly
> >> > wouldn't mind seeing this happen, but its more work than just adding a new
> >> > netlink message.
> >> >
> >> > Also, regarding XDP drops, we wont see them if the xdp program is offloaded to
> >> > hardware (you'll need your hw drop gathering mechanism for that), but for xdp
> >> > programs run on the cpu, dropwatch should alrady catch those.  I.e. if the xdp
> >> > program returns a DROP result for a packet being processed, the OS will call
> >> > kfree_skb on its behalf, and dropwatch wil call that.
> >> 
> >> There is no skb by the time an XDP program runs, so this is not true. As
> >> I mentioned upthread, there's a tracepoint that will get called if an
> >> error occurs (or the program returns XDP_ABORTED), but in most cases,
> >> XDP_DROP just means that the packet silently disappears...
> >> 
> > As I noted, thats only true for xdp programs that are offloaded to hardware, I
> > was only speaking for XDP programs that run on the cpu.  For the former case, we
> > obviously need some other mechanism to detect drops, but for cpu executed xdp
> > programs, the OS is responsible for freeing skbs associated with programs the
> > return XDP_DROP.
> 
> Ah, I think maybe you're thinking of generic XDP (also referred to as
> skb mode)? That is a separate mode; an XDP program loaded in "native
Yes, was I not clear about that?
Neil

> mode" (or "driver mode") runs on the CPU, but before the skb is created;
> this is the common case for XDP, and there is no skb and thus no drop
> notification in this mode.
> 
> There is *also* an offload mode for XDP programs, but that is only
> supported by netronome cards thus far, so not as commonly used...
> 
> -Toke
> 

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