Neil Horman <nhor...@tuxdriver.com> writes: > On Fri, Jul 12, 2019 at 11:27:55AM +0200, Toke Høiland-Jørgensen wrote: >> Neil Horman <nhor...@tuxdriver.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 <ido...@idosch.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 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
