Supporting PMUs on RISC-V platforms

Alan Kao <alankao@andestech.com>, Mar 2018

Introduction

As of this writing, perf_event-related features mentioned in The RISC-V ISA Privileged Version 1.10 are as follows: (please check the manual for more details)

  • [m|s]counteren
  • mcycle[h], cycle[h]
  • minstret[h], instret[h]
  • mhpeventx, mhpcounterx[h]

With such function set only, porting perf would require a lot of work, due to the lack of the following general architectural performance monitoring features:

  • Enabling/Disabling counters Counters are just free-running all the time in our case.
  • Interrupt caused by counter overflow No such feature in the spec.
  • Interrupt indicator It is not possible to have many interrupt ports for all counters, so an interrupt indicator is required for software to tell which counter has just overflowed.
  • Writing to counters There will be an SBI to support this since the kernel cannot modify the counters [1]. Alternatively, some vendor considers to implement hardware-extension for M-S-U model machines to write counters directly.

This document aims to provide developers a quick guide on supporting their PMUs in the kernel. The following sections briefly explain perf’ mechanism and todos.

You may check previous discussions here [1][2]. Also, it might be helpful to check the appendix for related kernel structures.

1. Initialization

riscv_pmu is a global pointer of type struct riscv_pmu, which contains various methods according to perf’s internal convention and PMU-specific parameters. One should declare such instance to represent the PMU. By default, riscv_pmu points to a constant structure riscv_base_pmu, which has very basic support to a baseline QEMU model.

Then he/she can either assign the instance’s pointer to riscv_pmu so that the minimal and already-implemented logic can be leveraged, or invent his/her own riscv_init_platform_pmu implementation.

In other words, existing sources of riscv_base_pmu merely provide a reference implementation. Developers can flexibly decide how many parts they can leverage, and in the most extreme case, they can customize every function according to their needs.

2. Event Initialization

When a user launches a perf command to monitor some events, it is first interpreted by the userspace perf tool into multiple perf_event_open system calls, and then each of them calls to the body of event_init member function that was assigned in the previous step. In riscv_base_pmu’s case, it is riscv_event_init.

The main purpose of this function is to translate the event provided by user into bitmap, so that HW-related control registers or counters can directly be manipulated. The translation is based on the mappings and methods provided in riscv_pmu.

Note that some features can be done in this stage as well:

  1. interrupt setting, which is stated in the next section;

  2. privilege level setting (user space only, kernel space only, both);

  3. destructor setting. Normally it is sufficient to apply riscv_destroy_event;

  4. tweaks for non-sampling events, which will be utilized by functions such as perf_adjust_period, usually something like the follows:

    if (!is_sampling_event(event)) {
        hwc->sample_period = x86_pmu.max_period;
        hwc->last_period = hwc->sample_period;
        local64_set(&hwc->period_left, hwc->sample_period);
}

In the case of riscv_base_pmu, only (3) is provided for now.

3. Interrupt

3.1. Interrupt Initialization

This often occurs at the beginning of the event_init method. In common practice, this should be a code segment like:

int x86_reserve_hardware(void)
{
      int err = 0;

      if (!atomic_inc_not_zero(&pmc_refcount)) {
              mutex_lock(&pmc_reserve_mutex);
              if (atomic_read(&pmc_refcount) == 0) {
                      if (!reserve_pmc_hardware())
                              err = -EBUSY;
                      else
                              reserve_ds_buffers();
              }
              if (!err)
                      atomic_inc(&pmc_refcount);
              mutex_unlock(&pmc_reserve_mutex);
      }

      return err;
}

And the magic is in reserve_pmc_hardware, which usually does atomic operations to make implemented IRQ accessible from some global function pointer. release_pmc_hardware serves the opposite purpose, and it is used in event destructors mentioned in previous section.

(Note: From the implementations in all the architectures, the reserve/release pair are always IRQ settings, so the pmc_hardware seems somehow misleading. It does NOT deal with the binding between an event and a physical counter, which will be introduced in the next section.)

3.2. IRQ Structure

Basically, a IRQ runs the following pseudo code:

for each hardware counter that triggered this overflow

    get the event of this counter

    // following two steps are defined as *read()*,
    // check the section Reading/Writing Counters for details.
    count the delta value since previous interrupt
    update the event->count (# event occurs) by adding delta, and
               event->hw.period_left by subtracting delta

    if the event overflows
        sample data
        set the counter appropriately for the next overflow

        if the event overflows again
            too frequently, throttle this event
        fi
    fi

end for

However as of this writing, none of the RISC-V implementations have designed an interrupt for perf, so the details are to be completed in the future.

4. Reading/Writing Counters

They seem symmetric but perf treats them quite differently. For reading, there is a read interface in struct pmu, but it serves more than just reading. According to the context, the read function not only reads the content of the counter (event->count), but also updates the left period to the next interrupt (event->hw.period_left).

But the core of perf does not need direct write to counters. Writing counters is hidden behind the abstraction of 1) pmu->start, literally start counting so one has to set the counter to a good value for the next interrupt; 2) inside the IRQ it should set the counter to the same resonable value.

Reading is not a problem in RISC-V but writing would need some effort, since counters are not allowed to be written by S-mode.

5. add()/del()/start()/stop()

Basic idea: add()/del() adds/deletes events to/from a PMU, and start()/stop() starts/stop the counter of some event in the PMU. All of them take the same arguments: struct perf_event *event and int flag.

Consider perf as a state machine, then you will find that these functions serve as the state transition process between those states. Three states (event->hw.state) are defined:

  • PERF_HES_STOPPED: the counter is stopped
  • PERF_HES_UPTODATE: the event->count is up-to-date
  • PERF_HES_ARCH: arch-dependent usage … we don’t need this for now

A normal flow of these state transitions are as follows:

  • A user launches a perf event, resulting in calling to event_init.
  • When being context-switched in, add is called by the perf core, with a flag PERF_EF_START, which means that the event should be started after it is added. At this stage, a general event is bound to a physical counter, if any. The state changes to PERF_HES_STOPPED and PERF_HES_UPTODATE, because it is now stopped, and the (software) event count does not need updating.
    • start is then called, and the counter is enabled. With flag PERF_EF_RELOAD, it writes an appropriate value to the counter (check previous section for detail). Nothing is written if the flag does not contain PERF_EF_RELOAD. The state now is reset to none, because it is neither stopped nor updated (the counting already started)
  • When being context-switched out, del is called. It then checks out all the events in the PMU and calls stop to update their counts.
    • stop is called by del and the perf core with flag PERF_EF_UPDATE, and it often shares the same subroutine as read with the same logic. The state changes to PERF_HES_STOPPED and PERF_HES_UPTODATE, again.
    • Life cycle of these two pairs: add and del are called repeatedly as tasks switch in-and-out; start and stop is also called when the perf core needs a quick stop-and-start, for instance, when the interrupt period is being adjusted.

Current implementation is sufficient for now and can be easily extended to features in the future.