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authorZachary Leaf <zachary.leaf@arm.com>2022-05-12 02:26:00 -0500
committerDamjan Marion <dmarion@0xa5.net>2022-07-12 15:29:23 +0000
commit268d7be66b8b48a230e06de645e3a8b7de29d93c (patch)
tree098d15e0c3f6fa7864c3f581367d8d5bb80495fa /src/vcl
parentc7d43a5eb19f2acab900274432cfd0e136d6cb44 (diff)
perfmon: enable perfmon plugin for Arm
This patch enables statistics from the Arm PMUv3 through the perfmon plugin. In comparison to using the Linux "perf" tool, it allows obtaining direct, per node level statistics (rather than per thread). By accessing the PMU counter registers directly from userspace, we can avoid the overhead of using a read() system call and get more accurate and fine grained statistics about the running of individual nodes. A demo of perfmon on Arm can be found at: https://asciinema.org/a/egVNN1OF7JEKHYmfl5bpDYxfF *Important Note* Perfmon on Arm is dependent on and works only on Linux kernel versions of v5.17+ as this is when userspace access to Arm perf counters was included. On most Arm systems, a maximum of 7 PMU events can be configured at once - (6x PMU events + 1x CPU_CYCLE counter). If some perf counters are in use elsewhere by other applications, and there are insufficient counters remaining to open the bundle, the perf_event_open call will fail (provided the events are grouped with the group_fd param, which perfmon currently utilises). See arm/events.h for a list of PMUv3 events available, although it is implementation defined whether most events are implemented or not. Only a small set of 7 events is required to be implemented in Armv8.0, with some additional events required in later versions. As such, depending on the implementation, some statistics may not be available. See Arm Architecture Reference Manual for Armv8-A, D7.10.2 "The PMU event number space and common events" for more information. arm/events.c:arm_init() gets information from the sysfs about what events are implemented on a particular CPU at runtime. Arm's implementation of the perfmon source callback .bundle_support uses this information to disable unsupported events in a bundle, or in the case no events are supported, disable the entire bundle. Where a particular event in a bundle is not implemented, the statistic for that event is shown as '-' in the 'show perfmon statistics' cli output, by disabling the column. There is additional code in perfmon.c to only open events which are marked as implemented. Since we're only opening and reading events that are implemented, some extra logic is required in cli.c to re-align either perfmon_node_stats_t or perfmon_reading_t with the column headings configured in each bundle, taking into account disabled columns. Userspace access to perf counters is disabled by default, and needs to be enabled with 'sudo sysctl kernel/perf_user_access=1'. There is a check built into the Arm event source init function (arm/events.c:arm_init) to check that userspace reading of perf counters is enabled in the /proc/sys/kernel/perf_user_access file. If the above file does not exist, it means the kernel version is unsupported. Users without a supported kernel will see a warning message, and no Arm bundles will be registered to use in perfmon. Enabling/using plugin: - include the following in startup.conf: - plugins { plugin perfmon_plugin.so { enable } - 'show perfmon bundle [verbose]' - show available statistics bundles - 'perfmon start bundle <bundle-name>' - enable and start logging - 'perfmon stop' - stop logging - 'show perfmon statistics' - show output For a general guide on using and understanding Arm PMUv3 events, see https://community.arm.com/arm-community-blogs/b/tools-software-ides-blog/posts/arm-neoverse-n1-performance-analysis-methodology Type: feature Signed-off-by: Zachary Leaf <zachary.leaf@arm.com> Tested-by: Jieqiang Wang <jieqiang.wang@arm.com> Change-Id: I0620fe5b1bbe78842dfb1d0b6a060bb99e777651
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