I recently noticed a strange phenomenon on a Debian Stretch server running as a paravirtualized guest on a Xen host: top showed the CPU either be 100% idle or 100% stolen. User, system, nice and waiting times were stuck at 0%. I cross-checked with vmstat and it showed 0% for all cpu time counters. Both tools are getting their information from /proc/stat which looked like the following:
cpu 5322 0 4376 12720669 37879 0 59 1198368772563 0 0 cpu0 5322 0 4376 12720669 37879 0 59 1198368772563 0 0 ...
The third to last value is the steal time, denoting “stolen time, which is the time spent in other operating systems when running in a virtualized environment” [procfs(5)]. This value looked way too high and, in particular, it was counting backwards. So, if I wanted to put this system into production, some debugging was required before…