On 11.02.2019 1:34, Thomas Gleixner wrote: > On Thu, 7 Feb 2019, Alexey Budankov wrote: > > General note: Please stay in the 80 char limit for all of the text. Yes, sure. [PATCH v2 4/4] implements wrapping at 72 columns. > >> +The perf_events system call API [2]_ allocates file descriptors for every configured >> +PMU event. Open file descriptors are a per-process accountable resource governed >> +by the RLIMIT_NOFILE [11]_ limit (ulimit -n), which is usually derived from the login >> +shell process. When configuring Perf collection for a long list of events on a >> +large server system, this limit can be easily hit preventing required monitoring >> +configuration. > > I'd move this sentence into a different paragraph and keep those related to > RLIMIT_NOFILE together. Makes sense. Let's have these two paragraphs: Open file descriptors +++++++++++++++++++++ Memory allocation +++++++++++++++++ > >> ... RLIMIT_NOFILE limit can be increased on per-user basis modifying >> +content of the limits.conf file [12]_ on some systems. > > On some systems? Well, let's avoid this subtlety and have it like: 'RLIMIT_NOFILE limit can be increased on per-user basis modifying content of the limits.conf file [12]_ .' > >> Ordinarily, a Perf sampling session >> +(perf record) requires an amount of open perf_event file descriptors that is not >> +less than a number of monitored events multiplied by a number of monitored CPUs. > > s/a number of/the number of/ Accepted. > > The ordinary use case is: > > perf CMD pile-of-events PROCESS > > which does not specify the monitored CPUs at all. Then the number of file > descriptors is NR_EVENTS * NR_ONLINE_CPUS. > >> +An amount of memory available to user processes for capturing performance monitoring > > The amount ... Accepted. > >> +data is governed by the perf_event_mlock_kb [2]_ setting. This perf_event specific >> +resource setting defines overall per-cpu limits of memory allowed for mapping >> +by the user processes to execute performance monitoring. The setting essentially >> +extends the RLIMIT_MEMLOCK [11]_ limit, but only for memory regions mapped specially > > s/specially/specifically/ Accepted. > >> +for capturing monitored performance events and related data. >> + >> +For example, if a machine has eight cores and perf_event_mlock_kb limit is set >> +to 516 KiB, then a user process is provided with 516 KiB * 8 = 4128 KiB of memory >> +above the RLIMIT_MEMLOCK limit (ulimit -l) for perf_event mmap buffers. In particular, >> +this means that, if the user wants to start two or more performance monitoring >> +processes, the user is required to manually distribute available 4128 KiB between the > > distribute the available Accepted. > >> +monitoring processes, for example, using the --mmap-pages Perf record mode option. >> +Otherwise, the first started performance monitoring process allocates all available >> +4128 KiB and the other processes will fail to proceed due to the lack of memory. >> + >> +RLIMIT_MEMLOCK and perf_event_mlock_kb resource costraints are ignored for > > constraints. Accepted. > >> +processes with the CAP_IPC_LOCK capability. Thus, perf_events/Perf privileged users > > what means perf_events/Perf ? 'perf_events/Perf privileged users' refers to the paragraph about privileged users. 'perf_events/Perf' means exact combination of the kernel subsystem (perf_events) and the privileged Perf tool (Perf) executable that enables certain group of users with performance monitoring capabilities without scope limit. > >> +can be provided with memory above the constraints for perf_events/Perf performance >> +monitoring purpose by providing the Perf executable with CAP_IPC_LOCK capability. > > Thanks, > > tglx > Thanks, Alexey
