linux-kernel - Re: [PATCH v5 0/2] perf: riscv: Preliminary Perf Event Support on RISC-V

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Date:   Tue, 24 Apr 2018 18:15:49 -0700
From:   Atish Patra <atish.patra@....com>
To:     Palmer Dabbelt <palmer@...ive.com>
Cc:     "alankao@...estech.com" <alankao@...estech.com>,
        "albert@...ive.com" <albert@...ive.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "sols@...ive.com" <sols@...ive.com>,
        "corbet@....net" <corbet@....net>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "greentime@...estech.com" <greentime@...estech.com>,
        "nickhu@...estech.com" <nickhu@...estech.com>
Subject: Re: [PATCH v5 0/2] perf: riscv: Preliminary Perf Event Support on
 RISC-V

On 4/24/18 5:29 PM, Palmer Dabbelt wrote:
> On Tue, 24 Apr 2018 15:16:16 PDT (-0700), atish.patra@....com wrote:
>> On 4/24/18 12:44 PM, Palmer Dabbelt wrote:
>>> On Tue, 24 Apr 2018 12:27:26 PDT (-0700), atish.patra@....com wrote:
>>>> On 4/24/18 11:07 AM, Atish Patra wrote:
>>>>> On 4/19/18 4:28 PM, Alan Kao wrote:
>>>>> However, I got an rcu-stall for the test "47: Event times".
>>>>> # ./perf test -v 47
>>>> Got it working. The test tries to attach the event to CPU0 which doesn't
>>>> exist in HighFive Unleashed. Changing it to cpu1 works.
>>>>
>>>> diff --git a/tools/perf/tests/event-times.c b/tools/perf/tests/event-times.c
>>>> index 1a2686f..eb11632f 100644
>>>> --- a/tools/perf/tests/event-times.c
>>>> +++ b/tools/perf/tests/event-times.c
>>>> @@ -113,9 +113,9 @@ static int attach__cpu_disabled(struct perf_evlist
>>>> *evlist)
>>>>            struct cpu_map *cpus;
>>>>            int err;
>>>>
>>>> -       pr_debug("attaching to CPU 0 as enabled\n");
>>>> +       pr_debug("attaching to CPU 1 as disabled\n");
>>>>
>>>> -       cpus = cpu_map__new("0");
>>>> +       cpus = cpu_map__new("1");
>>>>            if (cpus == NULL) {
>>>>                    pr_debug("failed to call cpu_map__new\n");
>>>>                    return -1;
>>>> @@ -142,9 +142,9 @@ static int attach__cpu_enabled(struct perf_evlist
>>>> *evlist)
>>>>            struct cpu_map *cpus;
>>>>            int err;
>>>>
>>>> -       pr_debug("attaching to CPU 0 as enabled\n");
>>>> +       pr_debug("attaching to CPU 1 as enabled\n");
>>>>
>>>> -       cpus = cpu_map__new("0");
>>>> +       cpus = cpu_map__new("1");
>>>>            if (cpus == NULL) {
>>>>                    pr_debug("failed to call cpu_map__new\n");
>>>>                    return -1;
>>>>
>>>>
>>>> Palmer,
>>>> Would it be better to officially document it somewhere that CPU0 doesn't
>>>> exist in the HighFive Unleashed board ?
>>>> I fear that there will be other standard tests/code path that may fail
>>>> because of inherent assumption of cpu0 presence.
>>>
>>> I think the best way to fix this is to just have BBL (or whatever the
>>> bootloader is) renumber the CPUs so they're contiguous and begin with 0.
>>
>> Do you mean BBL will update the device tree that kernel eventually parse
>> and set the hart id?
>> Sounds good to me unless it acts as a big hack in future boot loaders.
> 
> Right now the machine-mode and supervisor-mode hart IDs are logically separate:
> the bootloader just provides the hart ID as a register argument when starting
> the kernel.

Yes.

   BBL already needs to enumerate the harts by looking through the
> device tree for various other reasons (at least to mask off the harts that
> Linux doesn't support), so it's not that much effort to just maintain a mapping
> from supervisor-mode hart IDs to machine-mode hart IDs.
> 

But Linux also parses the device tree to get hart ID in 
riscv_of_processor_hart(). This is used to setup the possible/present 
cpu map in setup_smp().

Thus, Linux also need to see a device tree with cpu0-3 instead of 
cpu1-4. Otherwise, present cpu map will be incorrect. Isn't it ?

> I have some patches floating around that do this, but appear to do it
> incorrectly enough that nothing boots so maybe I'm missing something that makes
> this complicated :).
> 

Just a wild guess: May be the because of the above reason ;)