We have identified the root cause of the issue and unfortunately it is completely unrelated to the recent events we have had although it has most likely contributed to the high load and increased the impact.
We've had an immediate response to our P1 request with GCP and the root cause of the issue has been identified as high usage across majority of client nodes by a process named "/home/kubernetes/bin/gcfsd" - This process is a GCP managed process providing virtual mounts to the servers.
The below line shows this process using around 8 cores of CPU (Peaking to 20 and 22 cores on other clients).
2114 root 20 0 23.5g 15.9g 21088 S 757.0 8.5 1962:35 /home/kubernetes/bin/gcfsd --mountpoint=/run/gcfsd/mnt --maxcontentcachesizemb=213 --maxlargefilescachesizemb=213 --layercachedir=/var/lib/containerd/io.containerd.snap+
We are working on an immediate mitigation action at the moment and another update will be provided within 10 minutes.