1*4f1223e8SApple OSS Distributions# CPU Counters 2*4f1223e8SApple OSS Distributions 3*4f1223e8SApple OSS DistributionsThe xnu subsystems that manage CPU performance counters. 4*4f1223e8SApple OSS Distributions 5*4f1223e8SApple OSS Distributions## Overview 6*4f1223e8SApple OSS Distributions 7*4f1223e8SApple OSS DistributionsCPU performance counters are hardware registers that count events of interest to efficient CPU execution. 8*4f1223e8SApple OSS DistributionsCounters that measure events closely correlated with each CPU's execution pipeline are managed by the Core Performance Monitoring Unit (CPMU). 9*4f1223e8SApple OSS DistributionsThe CPMU contains both fixed instructions and cycles counters, as well as configurable counters that can be programmed to count any of several hundred possible events. 10*4f1223e8SApple OSS DistributionsIn addition to the CPMU, the Last Level Cache (LLC) hosts the Uncore Performance Monitoring Unit (UPMU), which measures effects that aren't necessarily correlated to a single CPU. 11*4f1223e8SApple OSS DistributionsAll counters in the UPMU are configurable. 12*4f1223e8SApple OSS Distributions 13*4f1223e8SApple OSS DistributionsCounters are typically used in one of two ways: 14*4f1223e8SApple OSS Distributions 15*4f1223e8SApple OSS Distributions1. In "counting" mode, their counts are periodically queried and tallied up for a duration of interest. 16*4f1223e8SApple OSS Distributions2. In "sampling" mode, the counters are programmed to generate a Performance Monitor Interrupt (PMI) periodically, during which the currently running code can be sampled, like a time profiler. 17*4f1223e8SApple OSS Distributions 18*4f1223e8SApple OSS Distributions## Subsystems 19*4f1223e8SApple OSS Distributions 20*4f1223e8SApple OSS DistributionsThere are several subsystems that provide access to CPU counter hardware: 21*4f1223e8SApple OSS Distributions 22*4f1223e8SApple OSS Distributions- kpc: The Kernel Performance Counter system is the oldest subsystem and still manages the configurable CPMU counters. 23*4f1223e8SApple OSS DistributionsIt can use PMIs from these counters to trigger kperf samples and counter values can be recorded in kperf samples. 24*4f1223e8SApple OSS Distributions 25*4f1223e8SApple OSS Distributions- Monotonic: The Monotonic system provides access to the fixed CPMU counters with limited support for PMIs. 26*4f1223e8SApple OSS DistributionsAdditionally, the UPMU is entirely provided by a Monotonic dev node interface. 27*4f1223e8SApple OSS Distributions 28*4f1223e8SApple OSS Distributions- cpc: The CPU Performance Counter subsystem provides a policy layer on top of kpc and Monotonic to prevent malicious use of the hardware. 29*4f1223e8SApple OSS Distributions 30*4f1223e8SApple OSS DistributionsEventually, cpc will subsume kpc's and Monotonic's roles in the system. 31*4f1223e8SApple OSS Distributions 32*4f1223e8SApple OSS Distributions## Integrations 33*4f1223e8SApple OSS Distributions 34*4f1223e8SApple OSS Distributions- The Recount subsystem makes extensive use of the fixed CPMU counters to attribute CPU resources back to threads and processes. 35*4f1223e8SApple OSS Distributions 36*4f1223e8SApple OSS Distributions- Microstackshot telemetry is sampled periodically using the CPMU's cycle PMI trigger. 37*4f1223e8SApple OSS Distributions 38*4f1223e8SApple OSS Distributions- Stackshot includes cycles and instructions for each thread container in its kcdata. 39*4f1223e8SApple OSS Distributions 40*4f1223e8SApple OSS Distributions- The kperf profiling system can trigger samples of thread states and call stacks using CPMU PMIs, allowing it to sample thread states and call stacks. 41*4f1223e8SApple OSS DistributionsAnd CPU counter values can be sampled by kperf on other triggers, like timers or kdebug events. 42*4f1223e8SApple OSS Distributions 43*4f1223e8SApple OSS Distributions## See Also 44*4f1223e8SApple OSS Distributions 45*4f1223e8SApple OSS Distributions- <doc:recount> 46