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

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Message-ID: <mhng-0abe4f84-0dd3-437c-8cf2-b87def246796@palmer-si-x1c4>
Date:   Tue, 24 Apr 2018 17:29:51 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     atish.patra@....com
CC:     alankao@...estech.com, albert@...ive.com, peterz@...radead.org,
        mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, sols@...ive.com, corbet@....net,
        linux-riscv@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, greentime@...estech.com,
        nickhu@...estech.com
Subject:     Re: [PATCH v5 0/2] perf: riscv: Preliminary Perf Event Support on RISC-V

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.  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.

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 :).