Message-ID: <fd91b8b2-7e6e-42b6-812d-3cf3ead882d3@linux.intel.com>
Date: Thu, 9 Oct 2025 10:31:08 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: "Falcon, Thomas" <thomas.falcon@...el.com>,
 "alexander.shishkin@...ux.intel.com" <alexander.shishkin@...ux.intel.com>,
 "peterz@...radead.org" <peterz@...radead.org>,
 "acme@...nel.org" <acme@...nel.org>, "mingo@...hat.com" <mingo@...hat.com>,
 "Hunter, Adrian" <adrian.hunter@...el.com>,
 "namhyung@...nel.org" <namhyung@...nel.org>,
 "jolsa@...nel.org" <jolsa@...nel.org>,
 "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
 "irogers@...gle.com" <irogers@...gle.com>,
 "mark.rutland@....com" <mark.rutland@....com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
 "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [RESEND][PATCH v2 0/2] perf record: ratio-to-prev event term for
 auto counter reload


On 10/2/2025 11:38 PM, Falcon, Thomas wrote:
> On Tue, 2025-09-30 at 15:28 +0800, Mi, Dapeng wrote:
>> On 9/3/2025 12:40 AM, Thomas Falcon wrote:
>>> The Auto Counter Reload (ACR)[1] feature is used to track the
>>> relative rates of two or more perf events, only sampling
>>> when a given threshold is exceeded. This helps reduce overhead
>>> and unnecessary samples. However, enabling this feature
>>> currently requires setting two parameters:
>>>
>>>  -- Event sampling period ("period")
>>>  -- acr_mask, which determines which events get reloaded
>>>     when the sample period is reached.
>>>
>>> For example, in the following command:
>>>
>>> perf record -e "{cpu_atom/branch-misses,period=200000,\
>>> acr_mask=0x2/ppu,cpu_atom/branch-instructions,period=1000000,\
>>> acr_mask=0x3/u}" -- ./mispredict
>>>
>>> The goal is to limit event sampling to cases when the
>>> branch miss rate exceeds 20%. If the branch instructions
>>> sample period is exceeded first, both events are reloaded.
>>> If branch misses exceed their threshold first, only the
>>> second counter is reloaded, and a sample is taken.
>>>
>>> To simplify this, provide a new “ratio-to-prev” event term
>>> that works alongside the period event option or -c option.
>>> This would allow users to specify the desired relative rate
>>> between events as a ratio, making configuration more intuitive.
>>>
>>> With this enhancement, the equivalent command would be:
>>>
>>> perf record -e "{cpu_atom/branch-misses/ppu,\
>>> cpu_atom/branch-instructions,period=1000000,ratio_to_prev=5/u}" \
>>> -- ./mispredict
>> Hi Tom,
>>
>> Does this "ratio-to-prev" option support 3 and more events in ACR
>> group?
>>
> Hi Dapeng,
>
> The 'ratio-to-prev' option only supports groups with two events at this
> time. For larger event groups, the "acr_mask" term is available.
>
>> If not, should we consider to support the cases there are 3 and more
>> events
>> in the ACR group? (If I remember correct, the PMU driver should
>> support it).
>>
> Correct.
>
>> e.g.,
>>
>> perf record -e
>> "{cpu_atom/branch-
>> misses,period=200000,acr_mask=0x6/p,cpu_atom/branches,period=1000000,
>> acr_mask=0x7/,cpu_atom/branches,period=1000000,acr_mask=0x7/}"
>> -- sleep 1
>>
>> Of course, this is just an example that indicates the cases are
>> supported,
>> it doesn't mean the command is meaningful. But we can't exclude that
>> users
>> have such real requirements.
>>
>> If we want to support 3 and more events in ACR group (if not
>> already), we'd
>> better rename the "ratio-to-prev" option to "ratio-to-head" and only
>> allow
>> the group leader can be set the sampling period explicitly with
>> "period"
>> option and the sampling period of all other group members can only be
>> calculated base on the sampling period of group leader and
>> the "ratio-to-head", maybe like this.
>>
>> perf record -e
>> "{cpu_atom/branch-misses,period=200000/p,cpu_atom/branches,ratio-to-
>> head=5/,cpu_atom/branches,ratio-to-head=5/}"
>> -- sleep 1
>>
>> Thanks.
>>
>>
> Thanks, those are good suggestions, but the goal of the feature was to
> provide users a way to utilize ACR to make simple comparisons without
> needing to use the "acr_mask" field. For tests comparing larger event
> groups, the acr_mask field may be used instead.

Yeah, I understand the intent is to get a new simple option for using the
ACR feature. But we already support 2 events for the ACR group, why not we
make it more generic and can support more events? 


>
> Thanks,
> Tom
>
>>> or
>>>
>>> perf record -e "{cpu_atom/branch-misses/ppu,\
>>> cpu_atom/branch-instructions,ratio-to-prev=5/u}" -c 1000000 \
>>> -- ./mispredict
>>>
>>> [1]
>>> https://lore.kernel.org/lkml/20250327195217.2683619-1-kan.liang@linux.intel.com/
>>>
>>> Changes in v2 (mostly suggested by Ian Rogers):
>>>
>>> -- Add documentation explaining acr_mask bitmask used by ACR
>>> -- Move ACR specific implementation to arch/x86/
>>> -- Provide test cases for event parsing and perf record tests
>>>
>>> Thomas Falcon (2):
>>>   perf record: Add ratio-to-prev term
>>>   perf record: add auto counter reload parse and regression tests
>>>
>>>  tools/perf/Documentation/intel-acr.txt | 53 ++++++++++++++++++
>>>  tools/perf/Documentation/perf-list.txt |  2 +
>>>  tools/perf/arch/x86/util/evsel.c       | 53 ++++++++++++++++++
>>>  tools/perf/tests/parse-events.c        | 54 ++++++++++++++++++
>>>  tools/perf/tests/shell/record.sh       | 40 ++++++++++++++
>>>  tools/perf/util/evsel.c                | 76
>>> ++++++++++++++++++++++++++
>>>  tools/perf/util/evsel.h                |  1 +
>>>  tools/perf/util/evsel_config.h         |  1 +
>>>  tools/perf/util/parse-events.c         | 22 ++++++++
>>>  tools/perf/util/parse-events.h         |  3 +-
>>>  tools/perf/util/parse-events.l         |  1 +
>>>  tools/perf/util/pmu.c                  |  3 +-
>>>  12 files changed, 307 insertions(+), 2 deletions(-)
>>>  create mode 100644 tools/perf/Documentation/intel-acr.txt
>>>

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