我可以在 PMC 溢出时获得用户空间中断或其他指示吗

Question

我可以配置perf 以便在 PMC 溢出时收到用户空间中断或其他通知吗？

也就是说，通常当 PMC 溢出时，内核会处理更新计数器、采样或其他需要完成的簿记，但我很感兴趣在用户空间接收这些并采取一些行动。

Answer 1

根据man 2 perf_event_open，“溢出处理”部分有perf_event_attr.watermark=0; perf_event_attr.wakeup_events=1 模式，它可能会在fd 上生成事件或为每个添加到perf_events 的rind 缓冲区的样本调用信号处理程序（在perf record 中的采样模式下）

 watermark    If set ...  Otherwise, overflow notifications happen 
              after wakeup_events samples.
 wakeup_events
              This union sets how many samples (wakeup_events) happen
              before an overflow notification happens.  

              wakeup_events counts only PERF_RECORD_SAMPLE record types.  To
              receive overflow notification for all PERF_RECORD types choose
              watermark and set wakeup_watermark to 1.

   Overflow handling

   Events can be set to notify when a threshold is crossed, indicating
   an overflow.  Overflow conditions can be captured by monitoring the
   event file descriptor with poll(2), select(2), or epoll(7).  Alterna‐
   tively, the overflow events can be captured via sa signal handler, by
   enabling I/O signaling on the file descriptor; see the discussion of
   the F_SETOWN and F_SETSIG operations in fcntl(2).

   Overflows are generated only by sampling events (sample_period must
   have a nonzero value).

   There are two ways to generate overflow notifications.

   The first is to set a wakeup_events or wakeup_watermark value that
   will trigger if a certain number of samples or bytes have been writ‐
   ten to the mmap ring buffer.  In this case, POLL_IN is indicated.

   The other way is by use of the PERF_EVENT_IOC_REFRESH ioctl.  This
   ioctl adds to a counter that decrements each time the event over‐
   flows.  When nonzero, POLL_IN is indicated, but once the counter
   reaches 0 POLL_HUP is indicated and the underlying event is disabled.

有类似但不精确的方法来限制环形缓冲区的大小，例如使用 perf 记录的-m 1 选项（工具将在大约每 100 个样本时发出环形填充信号，并在 fd 上进行轮询）。这对于查看实际的 perf_event_open 参数也很有用。

 $ strace -tttT -v perf record -m 1 -e cycles:u -F 20000 python -c 'print(1)' 2>&1 |less

还检查示例 https://mirrors.edge.kernel.org/pub/linux/kernel/tools/perf/ - 任何最近的 perf tar.gz，然后是 tests/bp_signal.c 或 tests/bp_signal_overflow.c：

// SPDX-License-Identifier: GPL-2.0
 ...
        pe.sample_period = THRESHOLD;
        pe.sample_type = PERF_SAMPLE_IP;
        pe.wakeup_events = 1;
 ...
        ioctl(fd, PERF_EVENT_IOC_RESET, 0);
        ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

        for (i = 0; i < EXECUTIONS; i++)
                test_function();

        ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);