On 02/07/2021 11:08, Michal Hocko wrote: > On Fri 02-07-21 09:50:11, Krzysztof Kozlowski wrote: > [...] >> The questions: How accurate are now the cgroup counters? > > The precision depends on the number of CPUs the workload is running on > as we do a per-cpu charge caching to optimize the accounting. This is > MEMCG_CHARGE_BATCH (32) pages currently. You can learn more by checking > try_charge function (mm/memcontrol.c). This explains the 32 pages, thanks! > >> I understood they should charge only pages allocated by the process, so >> why mmap(4 kB) causes max_usage_in_bytes=132 kB? > > Please note that kernel allocations (marked by __GFP_ACCOUNT) are > accounted as well so this is not only about mmaped memory. > >> Why mmap(4 MB) causes max_usage_in_bytes=4 MB + 34 pages? > > The specific number will depend on the executing - e.g. use up all but 3 > pages from CPU0 batch and have 31 pages on another cpu. > >> What is being accounted there (stack guards?)? >> >> Or maybe the entire LTP test checking so carefully memcg limits is useless? > > Well, I haven't really checked details of those tests and their > objective but aiming for an absolute precision is not really something > that is very useful IMHO. We are very likely to do optimizations like > the one mentioned above as the runtime tends to be much more important > than to-the-page precision. > > Hope this clarifies this a bit. Yes, thanks! Best regards, Krzysztof
