systemd-bootchart fails with ENOENT for "/proc/schedstat" when run from initial ramdisk

[jamuir]

Executing systemd-bootchart from the initial ramdisk fails when systemd does its switch-root procedure.

This can be reproduced on Fedora 41 with an initial ramdisk updated to include systemd-bootchart.

The systemd-bootchart documentation does not mention if execution from the initial ramdisk is supported or not, but, internally, systemd-bootchart sets argv[0][0] = '@', so it seem like this was supported at one point (setting argv[0][0] = '@' is one way to survive the switch-root process killing spree).

The failure happens here:

https://github.com/systemd/systemd-bootchart/blob/a15bcafb60b9a24d866024953e9965316ba73eaf/src/store.c#L191C1-L194C71

I will provide an strace log and more detailed steps to reproduce below.

[jamuir]

strace log is attached.

strace-proc-schedstat.log

To prepare an initial ramdisk with systemd-bootchart (and strace), you can do this:

sudo -i
mkdir -p initrd/root
cd initrd/root
gunzip --stdout /boot/initramfs-6.11.4-301.fc41.aarch64.img | cpio --extract 
cd usr/lib/systemd
cp /usr/lib/systemd/systemd-bootchart .
# you can check that all required libs are already present
#   ldd /usr/lib/systemd/systemd-bootchart
cd ../..
cd bin
cp /usr/bin/strace .
# you will need to copy a few libs to support strace
#   ldd /usr/bin/strace
cd ../..
find . | cpio -o -H newc --file=../initramfs-xx.cpio
cd ..
gzip --stdout initramfs-xx.cpio > initramfs-xx.img
cp initramfs-xx.img /boot/initramfs-xx.img

Reboot and then edit the grub command to boot using the new initial ramdisk:

initrd ($root)/initramfs-xx.img

Also, add a kernel param to boot into the rd.emergency target (I also added enforcing=0):

$ xargs -n1 < /proc/cmdline 
BOOT_IMAGE=(hd0,gpt2)/vmlinuz-6.11.4-301.fc41.aarch64
root=/dev/mapper/fedora_vbox-root
ro
rd.lvm.lv=fedora_vbox/root
rhgb
enforcing=0
rd.emergency

In the ramdisk emergency shell, run bootchart and then exit to continue booting:

# strace -o /run/log/strace.log /usr/lib/systemd/systemd-bootchart &
# exit

When you login as normal, systemd-bootchart won't be running.

The strace log shows that systemd-bootchart failed attempting to read /proc/schedstat and then exited:

clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=0, tv_nsec=32804124}, NULL) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
--- SIGCONT {si_signo=SIGCONT, si_code=SI_USER, si_pid=1, si_uid=0} ---
restart_syscall(<... resuming interrupted clock_nanosleep ...>) = 0
lseek(4, 0, SEEK_SET)                   = 0
pread64(5, "nr_free_pages 478087\nnr_zone_ina"..., 4095, 0) = 3531
openat(AT_FDCWD, "/proc/schedstat", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
writev(2, [{iov_base="Unable to read schedstat: No suc"..., iov_len=51}, {iov_base="\n", iov_len=1}], 2) = -1 EIO (Input/output error)
getpid()                                = 241
close(3)                                = 0
close(4)                                = 0
exit_group(1)                           = ?
+++ exited with 1 +++

You can also reproduce the defect by setting the kernel param rdinit=:

rdinit=/usr/lib/systemd/systemd-bootchart