perf_event_paranoid == 1 实际上对 x86 perf 有什么限制？

Question

较新的 Linux 内核有一个 sysfs 可调参数 /proc/sys/kernel/perf_event_paranoid 允许用户为非 root 用户调整 perf_events 的可用功能，数字越大越安全（提供功能相应减少）：

从 kernel documenation 我们对各种值有以下行为：

perf_event_paranoid:

Controls use of the performance events system by unprivileged users (without CAP_SYS_ADMIN). The default value is 2.

-1: Allow use of (almost) all events by all users Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK

>=0: Disallow ftrace function tracepoint by users without CAP_SYS_ADMIN Disallow raw tracepoint access by users without CAP_SYS_ADMIN

>=1: Disallow CPU event access by users without CAP_SYS_ADMIN

>=2: Disallow kernel profiling by users without CAP_SYS_ADMIN

我的 perf_event_paranoid 文件中有 1，应该 "Disallow CPU event access" - 但那到底是什么意思？

简单的阅读意味着无法访问 CPU 性能计数器事件（例如英特尔 PMU 事件），但我似乎可以很好地访问这些事件。例如：

$ perf stat sleep 1

 Performance counter stats for 'sleep 1':

          0.408734      task-clock (msec)         #    0.000 CPUs utilized          
                 1      context-switches          #    0.002 M/sec                  
                 0      cpu-migrations            #    0.000 K/sec                  
                57      page-faults               #    0.139 M/sec                  
         1,050,362      cycles                    #    2.570 GHz                    
           769,135      instructions              #    0.73  insn per cycle         
           152,661      branches                  #  373.497 M/sec                  
             6,942      branch-misses             #    4.55% of all branches        

       1.000830821 seconds time elapsed

这里，很多事件都是CPUPMU事件（cycles、instructions、branches、branch-misses、cache-misses） .

如果这些不是所指的 CPU 事件，它们是什么？

Answer 1

在这种情况下，CPU 事件 是指每个 CPU 而不是每个任务的监视事件。对于 perf 工具，这限制了

的使用

-C, --cpu=
    Count only on the list of CPUs provided. Multiple CPUs can be provided as a comma-separated list with no space: 0,1.
    Ranges of CPUs are specified with -: 0-2. In per-thread mode, this option is ignored. The -a option is still necessary
    to activate system-wide monitoring. Default is to count on all CPUs.

-a, --all-cpus
    system-wide collection from all CPUs (default if no target is specified)

对于 perf_event_open 这考虑了以下情况：

pid == -1 and cpu >= 0
       This measures all processes/threads on the specified CPU.  This requires CAP_SYS_ADMIN capability or a /proc/sys/ker‐
       nel/perf_event_paranoid value of less than 1.

这可能是特定于版本的，引用的文档来自 4.17。这是 another related question.

perf_event_paranoid == 1 实际上对 x86 perf 有什么限制？

What restriction is perf_event_paranoid == 1 actually putting on x86 perf?

x86

profiling

linux-kernel

perf

intel-pmu