[PATCH V4 0/5] Support auto counter reload

[kan.liang@linux.intel.com]

From: Kan Liang <kan.liang@linux.intel.com>

Changes since V3:
- Add static_call() for intel_pmu_enable_acr()
- Factor the repeated functions for acr in hw_config().
- Add Tested-by from Tom
- Rebase on top of Peter's perf/core
 commit 12e766d16814 (perf: Fix __percpu annotation)
V3 can be found at
https://lore.kernel.org/lkml/20250213211718.2406744-1-kan.liang@linux.intel.com/

Changes since V2:
- Rebase on top of several new features, e.g., counters snapshotting
  feature. Rewrite the code for the ACR CPUID-enumeration, configuration
  and late setup.
- Patch 1-3 are newly added for clean up.

Changes since V1:
- Add a check to the reload value which cannot exceeds the max period
- Avoid invoking intel_pmu_enable_acr() for the perf metrics event.
- Update comments explain to case which the event->attr.config2 exceeds
  the group size

The relative rates among two or more events are useful for performance
analysis, e.g., a high branch miss rate may indicate a performance
issue. Usually, the samples with a relative rate that exceeds some
threshold are more useful. However, the traditional sampling takes
samples of events separately. To get the relative rates among two or
more events, a high sample rate is required, which can bring high
overhead. Many samples taken in the non-hotspot area are also dropped
(useless) in the post-process.

The auto counter reload (ACR) feature takes samples when the relative
rate of two or more events exceeds some threshold, which provides the
fine-grained information at a low cost.
To support the feature, two sets of MSRs are introduced. For a given
counter IA32_PMC_GPn_CTR/IA32_PMC_FXm_CTR, bit fields in the
IA32_PMC_GPn_CFG_B/IA32_PMC_FXm_CFG_B MSR indicate which counter(s)
can cause a reload of that counter. The reload value is stored in the
IA32_PMC_GPn_CFG_C/IA32_PMC_FXm_CFG_C.
The details can be found at Intel SDM (085), Volume 3, 21.9.11 Auto
Counter Reload.

Example:

Here is the snippet of the mispredict.c. Since the array has a random
numbers, jumps are random and often mispredicted.
The mispredicted rate depends on the compared value.

For the Loop1, ~11% of all branches are mispredicted.
For the Loop2, ~21% of all branches are mispredicted.

main()
{
...
        for (i = 0; i < N; i++)
                data[i] = rand() % 256;
...
        /* Loop 1 */
        for (k = 0; k < 50; k++)
                for (i = 0; i < N; i++)
                        if (data[i] >= 64)
                                sum += data[i];
...

...
        /* Loop 2 */
        for (k = 0; k < 50; k++)
                for (i = 0; i < N; i++)
                        if (data[i] >= 128)
                                sum += data[i];
...
}

Usually, a code with a high branch miss rate means a bad performance.
To understand the branch miss rate of the codes, the traditional method
usually samples both branches and branch-misses events. E.g.,
perf record -e "{cpu_atom/branch-misses/ppu, cpu_atom/branch-instructions/u}"
               -c 1000000 -- ./mispredict

[ perf record: Woken up 4 times to write data ]
[ perf record: Captured and wrote 0.925 MB perf.data (5106 samples) ]
The 5106 samples are from both events and spread in both Loops.
In the post-process stage, a user can know that the Loop 2 has a 21%
branch miss rate. Then they can focus on the samples of branch-misses
events for the Loop 2.

With this patch, the user can generate the samples only when the branch
miss rate > 20%. For example,
perf record -e "{cpu_atom/branch-misses,period=200000,acr_mask=0x2/ppu,
                 cpu_atom/branch-instructions,period=1000000,acr_mask=0x3/u}"
                -- ./mispredict

(Two different periods are applied to branch-misses and
branch-instructions. The ratio is set to 20%.
If the branch-instructions is overflowed first, the branch-miss
rate < 20%. No samples should be generated. All counters should be
automatically reloaded.
If the branch-misses is overflowed first, the branch-miss rate > 20%.
A sample triggered by the branch-misses event should be
generated. Just the counter of the branch-instructions should be
automatically reloaded.

The branch-misses event should only be automatically reloaded when
the branch-instructions is overflowed. So the "cause" event is the
branch-instructions event. The acr_mask is set to 0x2, since the
event index in the group of branch-instructions is 1.

The branch-instructions event is automatically reloaded no matter which
events are overflowed. So the "cause" events are the branch-misses
and the branch-instructions event. The acr_mask should be set to 0x3.)

[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.098 MB perf.data (2498 samples) ]

 $perf report

Percent       │154:   movl    $0x0,-0x14(%rbp)
              │     ↓ jmp     1af
              │     for (i = j; i < N; i++)
              │15d:   mov     -0x10(%rbp),%eax
              │       mov     %eax,-0x18(%rbp)
              │     ↓ jmp     1a2
              │     if (data[i] >= 128)
              │165:   mov     -0x18(%rbp),%eax
              │       cltq
              │       lea     0x0(,%rax,4),%rdx
              │       mov     -0x8(%rbp),%rax
              │       add     %rdx,%rax
              │       mov     (%rax),%eax
              │    ┌──cmp     $0x7f,%eax
100.00   0.00 │    ├──jle     19e
              │    │sum += data[i];

The 2498 samples are all from the branch-misses events for the Loop 2.

The number of samples and overhead is significantly reduced without
losing any information.

Kan Liang (5):
  perf/x86: Add dynamic constraint
  perf/x86/intel: Track the num of events needs late setup
  perf: Extend the bit width of the arch-specific flag
  perf/x86/intel: Add CPUID enumeration for the auto counter reload
  perf/x86/intel: Support auto counter reload

 arch/x86/events/core.c             |   3 +-
 arch/x86/events/intel/core.c       | 267 ++++++++++++++++++++++++++++-
 arch/x86/events/intel/ds.c         |   3 +-
 arch/x86/events/intel/lbr.c        |   2 +-
 arch/x86/events/perf_event.h       |  33 ++++
 arch/x86/events/perf_event_flags.h |  41 ++---
 arch/x86/include/asm/msr-index.h   |   4 +
 arch/x86/include/asm/perf_event.h  |   1 +
 include/linux/perf_event.h         |   4 +-
 9 files changed, 327 insertions(+), 31 deletions(-)

-- 
2.38.1