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perf-security: document perf_events/Perf resource control 

Extend perf-security.rst file with perf_events/Perf resource control
section describing RLIMIT_NOFILE and perf_event_mlock_kb settings for
performance monitoring user processes.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Alexey Budankov 2019-02-11 16:42:58 +03:00 committed by Jonathan Corbet
parent 44a47f0e3e
commit 9d87bbae2d
1 changed files with 42 additions and 0 deletions
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Documentation/admin-guide/perf-security.rst
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@ -84,6 +84,46 @@ governed by perf_event_paranoid [2]_ setting:
locking limit is imposed but ignored for unprivileged processes with
CAP_IPC_LOCK capability.
perf_events/Perf resource control
---------------------------------
Open file descriptors
+++++++++++++++++++++
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. 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 the number of monitored events multiplied by the number of monitored CPUs.
Memory allocation
+++++++++++++++++
The amount of memory available to user processes for capturing performance monitoring
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 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 the available 4128 KiB between the
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 constraints are ignored for
processes with the CAP_IPC_LOCK capability. Thus, perf_events/Perf privileged users
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.
Bibliography
------------
@ -94,4 +134,6 @@ Bibliography
.. [5] `<https://www.kernel.org/doc/html/latest/security/credentials.html>`_
.. [6] `<http://man7.org/linux/man-pages/man7/capabilities.7.html>`_
.. [7] `<http://man7.org/linux/man-pages/man2/ptrace.2.html>`_
.. [11] `<http://man7.org/linux/man-pages/man2/getrlimit.2.html>`_
.. [12] `<http://man7.org/linux/man-pages/man5/limits.conf.5.html>`_