Re: Expensive memory.stat + cpu.stat reads

[Shakeel Butt <shakeelb@google.com>]

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