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Ido Schimmel says: ==================== drop_monitor: Capture dropped packets and metadata So far drop monitor supported only one mode of operation in which a summary of recent packet drops is periodically sent to user space as a netlink event. The event only includes the drop location (program counter) and number of drops in the last interval. While this mode of operation allows one to understand if the system is dropping packets, it is not sufficient if a more detailed analysis is required. Both the packet itself and related metadata are missing. This patchset extends drop monitor with another mode of operation where the packet - potentially truncated - and metadata (e.g., drop location, timestamp, netdev) are sent to user space as a netlink event. Thanks to the extensible nature of netlink, more metadata can be added in the future. To avoid performing expensive operations in the context in which kfree_skb() is called, the dropped skbs are cloned and queued on per-CPU skb drop list. The list is then processed in process context (using a workqueue), where the netlink messages are allocated, prepared and finally sent to user space. A follow-up patchset will integrate drop monitor with devlink and allow the latter to call into drop monitor to report hardware drops. In the future, XDP drops can be added as well, thereby making drop monitor the go-to netlink channel for diagnosing all packet drops. Example usage with patched dropwatch [1] can be found here [2]. Example dissection of drop monitor netlink events with patched wireshark [3] can be found here [4]. I will submit both changes upstream after the kernel changes are accepted. Another change worth making is adding a dropmon pseudo interface to libpcap, similar to the nflog interface [5]. This will allow users to specifically listen on dropmon traffic instead of capturing all netlink packets via the nlmon netdev. Patches #1-#5 prepare the code towards the actual changes in later patches. Patch #6 adds another mode of operation to drop monitor in which the dropped packet itself is notified to user space along with metadata. Patch #7 allows users to truncate reported packets to a specific length, in case only the headers are of interest. The original length of the packet is added as metadata to the netlink notification. Patch #8 allows user to query the current configuration of drop monitor (e.g., alert mode, truncation length). Patches #9-#10 allow users to tune the length of the per-CPU skb drop list according to their needs. Changes since v1 [6]: * Add skb protocol as metadata. This allows user space to correctly dissect the packet instead of blindly assuming it is an Ethernet packet Changes since RFC [7]: * Limit the length of the per-CPU skb drop list and make it configurable * Do not use the hysteresis timer in packet alert mode * Introduce alert mode operations in a separate patch and only then introduce the new alert mode * Use 'skb->skb_iif' instead of 'skb->dev' because the latter is inside a union with 'dev_scratch' and therefore not guaranteed to point to a valid netdev * Return '-EBUSY' instead of '-EOPNOTSUPP' when trying to configure drop monitor while it is monitoring * Did not change schedule_work() in favor of schedule_work_on() as I did not observe a change in number of tail drops [1] https://github.com/idosch/dropwatch/tree/packet-mode [2] https://gist.github.com/idosch/3d524b887e16bc11b4b19e25c23dcc23#file-gistfile1-txt [3] https://github.com/idosch/wireshark/tree/drop-monitor-v2 [4] https://gist.github.com/idosch/3d524b887e16bc11b4b19e25c23dcc23#file-gistfile2-txt [5] https://github.com/the-tcpdump-group/libpcap/blob/master/pcap-netfilter-linux.c [6] https://patchwork.ozlabs.org/cover/1143443/ [7] https://patchwork.ozlabs.org/cover/1135226/ ==================== Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.