On Fri, Jul 14, 2023 at 10:23 AM Waiman Long <longman@xxxxxxxxxx> wrote: > > On 7/13/23 19:25, Ivan Babrou wrote: > > 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 > > Yes, it is similar to what is being described in those articles. > > > > > > 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. > > The perf-c2c tool may be able to help. The data to look for is how often > the data is from caches vs direct memory load/store. It looks like c2c only works for the whole system, not individual treads. There's a lot of noise from the rest of the system.
