On 18 July 2018 at 16:47, Michal Hocko <mhocko@xxxxxxxxxx> wrote: >> Thanks for looking into this. I'm not familiar with ftrace. Can you >> give me a specific command line to run? Based on "perf record cat >> /sys/fs/cgroup/memory/memory.stat"/"perf report", I see the following: >> >> 42.09% cat [kernel.kallsyms] [k] memcg_stat_show >> 29.19% cat [kernel.kallsyms] [k] memcg_sum_events.isra.22 >> 12.41% cat [kernel.kallsyms] [k] mem_cgroup_iter >> 5.42% cat [kernel.kallsyms] [k] _find_next_bit >> 4.14% cat [kernel.kallsyms] [k] css_next_descendant_pre >> 3.44% cat [kernel.kallsyms] [k] find_next_bit >> 2.84% cat [kernel.kallsyms] [k] mem_cgroup_node_nr_lru_pages > > I would just use perf record as you did. How long did the call take? > Also is the excessive time an outlier or a more consistent thing? If the > former does perf record show any difference? I didn't note the exact time for that particular run, but it's pretty consistently 372-377ms on the machine that has that perf report. The times differ between machines showing the symptom (anywhere from 200-500ms), but are consistent (within a few ms) in back-to-back runs on each machine. >> Ubuntu 16.04 with kernel 4.13.0-41-generic (so presumably includes >> some Ubuntu special sauce). > > Do you see the same whe running with the vanilla kernel? We don't currently have any boxes running vanilla kernels. While I could install a test box with a vanilla kernel, I don't know how to reproduce the problem, what piece of our production environment is triggering it, or even why some machines are unaffected, so if the problem didn't re-occur on the test box I wouldn't be able to conclude anything useful. Do you have suggestions on things I could try that might trigger this? e.g. are there cases where a cgroup no longer shows up in the filesystem but is still lingering while waiting for its refcount to hit zero? Does every child cgroup contribute to the stat_show cost of its parent or does it have to have some non-trivial variation from its parent? Thanks Bruce -- Bruce Merry Senior Science Processing Developer SKA South Africa
