Re: [PATCH 7.1.y] Revert "tracing: perf: Fix stale head for perf syscall tracing"

[Alban Crequy <alban.crequy@gmail.com>]

Hi Sasha, Peter,

Another data point: Ubuntu picked up the same backport in its 7.0 kernel (7.0.0-38-generic), and it breaks Inspektor Gadget and bpftrace in the same way. A BPF program on sys_enter_execve or sys_enter_openat only runs on CPU 0; sys_exit_* and sys_enter_close keep working on all CPUs. Reproducer:

  bpftrace -e 'tracepoint:syscalls:sys_enter_openat /comm == "cat"/ { @[cpu] = count(); }' &
  for c in $(seq 0 7); do taskset -c $c cat /etc/hostname >/dev/null; done

Only @[0] shows up.

I also tested v7.1.13 under QEMU with 4 CPUs, using a small cilium/ebpf test that counts per-CPU hits of a tracepoint program while running 10 syscalls pinned to each CPU:

  v7.1.13:          sys_enter_openat [10 0 0 0], sys_enter_close [10 10 10 10], sys_exit_openat [10 10 10 10]
  v7.1.13 + revert: sys_enter_openat [10 10 10 10], sys_enter_close [10 10 10 10], sys_exit_openat [10 10 10 10]

The revert looks right to me for 7.1.y, and for any other stable branch without 57918341dd19.

Tested-by: Alban Crequy <albancrequy@microsoft.com>

Alban