diff options
Diffstat (limited to 'Documentation/riscv')
| -rw-r--r-- | Documentation/riscv/pmu.rst | 255 |
1 files changed, 0 insertions, 255 deletions
diff --git a/Documentation/riscv/pmu.rst b/Documentation/riscv/pmu.rst deleted file mode 100644 index acb216b99c26..000000000000 --- a/Documentation/riscv/pmu.rst +++ /dev/null @@ -1,255 +0,0 @@ -=================================== -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. - -A. Related Structures ---------------------- - -* struct pmu: include/linux/perf_event.h -* struct riscv_pmu: arch/riscv/include/asm/perf_event.h - - Both structures are designed to be read-only. - - *struct pmu* defines some function pointer interfaces, and most of them take - *struct perf_event* as a main argument, dealing with perf events according to - perf's internal state machine (check kernel/events/core.c for details). - - *struct riscv_pmu* defines PMU-specific parameters. The naming follows the - convention of all other architectures. - -* struct perf_event: include/linux/perf_event.h -* struct hw_perf_event - - The generic structure that represents perf events, and the hardware-related - details. - -* struct riscv_hw_events: arch/riscv/include/asm/perf_event.h - - The structure that holds the status of events, has two fixed members: - the number of events and the array of the events. - -References ----------- - -[1] https://github.com/riscv/riscv-linux/pull/124 - -[2] https://groups.google.com/a/groups.riscv.org/forum/#!topic/sw-dev/f19TmCNP6yA |