On Mon, Jul 10, 2023 at 5:44 PM Waiman Long <longman@xxxxxxxxxx> wrote: > > On 7/10/23 19:21, Ivan Babrou wrote: > > On Wed, Jul 5, 2023 at 11:20 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > >> 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? > > I'm afraid there's no easy way to reproduce. We see it from time to > > time in different locations. The one that I was looking at for the > > initial email does not reproduce it anymore: > > My understanding of mem-stat and cpu-stat is that they are independent > of each other. In theory, reading one shouldn't affect the performance > of reading the others. Since you are doing mem-stat and cpu-stat reading > repetitively in a loop, it is likely that all the data are in the cache > most of the time resulting in very fast processing time. If it happens > that the specific memory location of mem-stat and cpu-stat data are such > that reading one will cause the other data to be flushed out of the > cache and have to be re-read from memory again, you could see > significant performance regression. > > It is one of the possible causes, but I may be wrong. Do you think it's somewhat similar to how iterating a matrix in rows is faster than in columns due to sequential vs random memory reads? * https://stackoverflow.com/q/9936132 * https://en.wikipedia.org/wiki/Row-_and_column-major_order * https://en.wikipedia.org/wiki/Loop_interchange I've had a similar suspicion and it would be good to confirm whether it's that or something else. I can probably collect perf counters for different runs, but I'm not sure which ones I'll need. In a similar vein, if we could come up with a tracepoint that would tell us the amount of work done (or any other relevant metric that would help) during rstat flushing, I can certainly collect that information as well for every reading combination.
