Re: git: c1e813d12309 - main - hwpmc: Correct selection of Intel fixed counters.

Kyle, Andrew,

On 19.04.2023 16:06, Kyle Evans wrote:
> On Mon, May 30, 2022 at 7:05 PM Alexander Motin <mav@freebsd.org> wrote:
>>
>> The branch main has been updated by mav:
>>
>> URL: https://cgit.FreeBSD.org/src/commit/?id=c1e813d1230915e19a236ec687cadc1051841e56
>>
>> commit c1e813d1230915e19a236ec687cadc1051841e56
>> Author:     Alexander Motin <mav@FreeBSD.org>
>> AuthorDate: 2022-05-30 23:46:48 +0000
>> Commit:     Alexander Motin <mav@FreeBSD.org>
>> CommitDate: 2022-05-31 00:05:15 +0000
>>
>>      hwpmc: Correct selection of Intel fixed counters.
>>
>>      Intel json's use event=0 to specify fixed counter number via umask.
>>      Alternatively fixed counters have equivalent programmable event/umask.
>>
>>      MFC after:      1 month
>> ---
>>   sys/dev/hwpmc/hwpmc_core.c | 34 +++++++++++++++++++++++++---------
>>   1 file changed, 25 insertions(+), 9 deletions(-)
>>
>> diff --git a/sys/dev/hwpmc/hwpmc_core.c b/sys/dev/hwpmc/hwpmc_core.c
>> index fce97cbd5b8e..73cfee0b3975 100644
>> --- a/sys/dev/hwpmc/hwpmc_core.c
>> +++ b/sys/dev/hwpmc/hwpmc_core.c
>> @@ -245,15 +245,31 @@ iaf_allocate_pmc(int cpu, int ri, struct pmc *pm,
>>          ev = IAP_EVSEL_GET(config);
>>          umask = IAP_UMASK_GET(config);
>>
>> -       /* INST_RETIRED.ANY */
>> -       if (ev == 0xC0 && ri != 0)
>> -               return (EINVAL);
>> -       /* CPU_CLK_UNHALTED.THREAD */
>> -       if (ev == 0x3C && ri != 1)
>> -               return (EINVAL);
>> -       /* CPU_CLK_UNHALTED.REF */
>> -       if (ev == 0x0 && umask == 0x3 && ri != 2)
>> -               return (EINVAL);
>> +       if (ev == 0x0) {
>> +               if (umask != ri + 1)
>> +                       return (EINVAL);
> 
> Hey Alexander,
> 
> This seems to break system-mode sampling of fixed-mode counters; e.g.,
> from the manpage, `pmcstat -S instructions`, and I'm not at all
> familiar with these parts of pmc. We come in once with umask=1, ri=0;
> then again with umask=1, ri=1. The latter fails and we try umask=1,
> ri=2, which again fails, and the PMCALLOCATE op ultimately fails.
> 
> Is `umask != ri + 1` correct, or should this be reverted back to
> `umask == 0x3 && ri != 2` or something else? I've no idea what the
> `umask` values represent in this context.

I still believe this code is correct, at least as much as data in 
lib/libpmc/pmu-events/arch/x86 is consistent (look for 
"INST_RETIRED.ANY", that actually fails here after translation from 
"instructions").  The multiple calls you see is the result of hwpmc 
trying to find counter "matching" parameters.  For fixed counters the 
match is really just (umask == ri + 1), since umask for fixed counters 
in the Intel events data is just a counter number starting from 1.

I've tried to test fixed-mode counters myself and got it failing also. 
But looking a bit deeper, I think the problem may be caused by this 
patch: https://reviews.freebsd.org/D35709 .  I think it tries to 
allocate the same fixed counter twice, where the first attempt succeeds, 
while the second fails, since the hardware is already busy.  I haven't 
looked close on the pmcstat code, but I suppose the patch was wrong, so 
if you or Andrew wish to fix it properly -- I'd be happy.

-- 
Alexander Motin