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SubjectRe: [PATCH v2 1/4] perf-security: document perf_events/Perf resource control

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/


> 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 ...


>> +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/


>> +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


>> +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.


>> +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


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