Hi Jim,
On 03-Feb-22 9:39 AM, Jim Mattson wrote:
> On Wed, Feb 2, 2022 at 2:52 AM Ravi Bangoria <ravi.bangoria@xxxxxxx> wrote:
>>
>> Perf counter may overcount for a list of Retire Based Events. Implement
>> workaround for Zen3 Family 19 Model 00-0F processors as suggested in
>> Revision Guide[1]:
>>
>> To count the non-FP affected PMC events correctly:
>> o Use Core::X86::Msr::PERF_CTL2 to count the events, and
>> o Program Core::X86::Msr::PERF_CTL2[43] to 1b, and
>> o Program Core::X86::Msr::PERF_CTL2[20] to 0b.
>>
>> Note that the specified workaround applies only to counting events and
>> not to sampling events. Thus sampling event will continue functioning
>> as is.
>>
>> Although the issue exists on all previous Zen revisions, the workaround
>> is different and thus not included in this patch.
>>
>> This patch needs Like's patch[2] to make it work on kvm guest.
>
> IIUC, this patch along with Like's patch actually breaks PMU
> virtualization for a kvm guest.
>
> Suppose I have some code which counts event 0xC2 [Retired Branch
> Instructions] on PMC0 and event 0xC4 [Retired Taken Branch
> Instructions] on PMC1. I then divide PMC1 by PMC0 to see what
> percentage of my branch instructions are taken. On hardware that
> suffers from erratum 1292, both counters may overcount, but if the
> inaccuracy is small, then my final result may still be fairly close to
> reality.
>
> With these patches, if I run that same code in a kvm guest, it looks
> like one of those events will be counted on PMC2 and the other won't
> be counted at all. So, when I calculate the percentage of branch
> instructions taken, I either get 0 or infinity.
Events get multiplexed internally. See below quick test I ran inside
guest. My host is running with my+Like's patch and guest is running
with only my patch.
$ ./perf stat -e branch-instructions,branch-misses -- ./branch-misses
Performance counter stats for './branch-misses':
19,847,153,209 branch-instructions:u (50.03%)
950,410,251 branch-misses:u # 4.79% of all branches (49.97%)
$ cat branch-misses.c
#include <stdlib.h>
int main()
{
long i = 1000000000;
long j = 0;
while(i--) {
switch(rand() % 20) {
case 0: j += 0; break;
case 1: j += 1; break;
case 2: j += 2; break;
case 3: j += 3; break;
case 4: j += 4; break;
case 5: j += 5; break;
case 6: j += 6; break;
case 7: j += 7; break;
case 8: j += 8; break;
case 9: j += 9; break;
case 10: j += 10; break;
case 11: j += 11; break;
case 12: j += 12; break;
case 13: j += 13; break;
case 14: j += 14; break;
case 15: j += 15; break;
case 16: j += 16; break;
case 17: j += 17; break;
case 18: j += 18; break;
case 19: j += 19; break;
default: j += 20; break;
}
}
return 0;
}
Thanks,
Ravi
