On Fri, Jun 30, 2023 at 04:22:28PM -0700, Ivan Babrou wrote: > Hello, > > We're seeing CPU load issues with cgroup stats retrieval. I made a > public gist with all the details, including the repro code (which > unfortunately requires heavily loaded hardware) and some flamegraphs: > > * https://gist.github.com/bobrik/5ba58fb75a48620a1965026ad30a0a13 > > I'll repeat the gist of that gist here. Our repro has the following > output after a warm-up run: > > completed: 5.17s [manual / mem-stat + cpu-stat] > completed: 5.59s [manual / cpu-stat + mem-stat] > completed: 0.52s [manual / mem-stat] > completed: 0.04s [manual / cpu-stat] > > The first two lines do effectively the following: > > for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/memory.stat > /sys/fs/cgroup/system.slice/cpu.stat > /dev/null > > The latter two are the same thing, but via two loops: > > for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/cpu.stat > > /dev/null; done > for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/memory.stat > > /dev/null; done > > As you might've noticed from the output, splitting the loop into two > makes the code run 10x faster. This isn't great, because most > monitoring software likes to get all stats for one service before > reading the stats for the next one, which maps to the slow and > expensive way of doing this. > > We're running Linux v6.1 (the output is from v6.1.25) with no patches > that touch the cgroup or mm subsystems, so you can assume vanilla > kernel. > > From the flamegraph it just looks like rstat flushing takes longer. I > used the following flags on an AMD EPYC 7642 system (our usual pick > cpu-clock was blaming spinlock irqrestore, which was questionable): > > perf -e cycles -g --call-graph fp -F 999 -- /tmp/repro > > Naturally, there are two questions that arise: > > * Is this expected (I guess not, but good to be sure)? > * What can we do to make this better? > > I am happy to try out patches or to do some tracing to help understand > this better. Hi Ivan, Thanks a lot, as always, for reporting this. This is not expected and should be fixed. Is the issue easy to repro or some specific workload or high load/traffic is required? Can you repro this with the latest linus tree? Also do you see any difference of root's cgroup.stat where this issue happens vs good state? BTW I am away for next month with very limited connectivity, so expect slow response. thanks, Shakeel
