Is the TASK-PID in trace-cmd output the TID of the thread handling TAP interface I/O?

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virtual-machine qemu tracing virtio ftrace

                          I'm working on an networking lab tool leveraging QEMU-based VM virtualization and docker technology to run VMs and containers respectively on a Linux host. The underlying lab connectivity is implemented by using linux bridges.

I have a linux Ubuntu guest running inside a QEMU VM that features a virtio-net paravirualized interface with TAP backend. Such TAP interface is connected to a linux bridge's port on the host.

    root@eve-ng62-28:~# brctl show vnet0_3
    bridge name     bridge id               STP enabled     interfaces
    vnet0_3         8000.d63b1f37e4ba       no              vnet0_9_2
                                                            vunl0_3_3
                                                            vunl0_7_0
                                                            vunl0_9_2
    root@eve-ng62-28:~#
    root@eve-ng62-28:~# ethtool -i vunl0_7_0
    driver: tun
    version: 1.6
    firmware-version:
    expansion-rom-version:
    bus-info: tap
    supports-statistics: no
    supports-test: no
    supports-eeprom-access: no
    supports-register-dump: no
    supports-priv-flags: no
    root@eve-ng62-28:~#

I'm using Linux ftrace via trace-cmd frontend to dig into some details, see also https://unix.stackexchange.com/questions/797717/tcp-checksum-offloading-on-virtio-net-paravirtualized-interfaces 

    root@eve-ng62-28:~# trace-cmd start -e net:netif_receive_skb_entry -f "name == 'vunl0_7_0'"
    root@eve-ng62-28:~#
    root@eve-ng62-28:~# trace-cmd show
    # tracer: nop
    #
    # entries-in-buffer/entries-written: 1/1   #P:48
    #
    #                                _-----=> irqs-off/BH-disabled
    #                               / _----=> need-resched
    #                              | / _---=> hardirq/softirq
    #                              || / _--=> preempt-depth
    #                              ||| / _-=> migrate-disable
    #                              |||| /     delay
    #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
    #              | |         |   |||||     |         |
    qemu-system-x86-600348   b.... 66505.777999: netif_receive_skb_entry: dev=vunl0_7_0 napi_id=0x0 queue_mapping=1 skbaddr=0000000006a1cc35 vlan_tagged=0 vlan_proto=0x0000 vlan_tci=0x0000 protocol=0x0800 ip_summed=3 hash=0x00000000 l4_hash=0 len=60 data_len=0 truesize=768 mac_header_valid=1 mac_header=-14 nr_frags=0 gso_size=0 gso_type=0x0

As you can see, linux guest sends outgoing TCP packets to the virtio-net network interface setting CHECKSUM_PARTIAL(3) tag in the ip_summed field within sk_buff struct. 

My question is related to the TASK-PID field shown by trace-cmd show. 600348 is the PID of the qemu-system-x86_64 process's instance associated to the VM.

As required I edit this to provide the question: is the TASK-PID shown the PID or TID of the process/thread that is the context the TAP driver runs into ?
                        

Asked by CarloC (385 rep)

Jul 9, 2025, 01:03 PM
Last activity: Jul 12, 2025, 07:42 PM

Is the TASK-PID in trace-cmd output the TID of the thread handling TAP interface I/O?

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