Every CPU reading on this failure path describes a subject inside the
sandbox -- a cgroup, a QEMU thread, a node's own /proc/stat -- and all of
them can report a guest that was handed its ticks while the guest gets no
work done. None of them can say what a tick cost. That price is paid one
layer down: a tenant worker is a guest of a sandbox node, which is itself
a guest of the runner, and the runner's kernel absorbs the exits, nested
page faults and TLB flushes the arrangement provokes.
Read those counters from the debugfs that kernel publishes them in. The
suite runs in a container sharing it, so this is a local read rather than
a walk over nodes, which is what makes it affordable at all: the phase
budget has no room left for a further per-node read. debugfs is mounted
in a mount namespace belonging to the call, so nothing is left behind for
the twenty minutes of suite that follow it.
The pair is taken across the node-join wait rather than inside the
diagnostics block: once before the wait on every run, once after it on
the failing path and on the passing one. The counters are running
totals -- per-VM since that VM was created, kernel-wide summed over the
VMs alive at the read -- so the interval a difference divides by becomes
the window the guest was failing to boot in, and a passing run leaves a
baseline to read the failing ones against.
Assisted-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <3811295@gmail.com>
Aleksei Sviridkin
827f4e7e10