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Message-ID: <e7e0ef26-2335-4e67-984c-705cb33ff4c3@linux.intel.com>
Date: Thu, 5 Sep 2024 16:29:19 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-perf-users@...r.kernel.org,
peterz@...radead.org, x86@...nel.org, mingo@...hat.com,
linux-kernel@...r.kernel.org, bpf@...r.kernel.org, acme@...nel.org,
kernel-team@...a.com, stable@...r.kernel.org
Subject: Re: [PATCH] perf/x86: fix wrong assumption that LBR is only useful
for sampling events
On 2024-09-05 4:22 p.m., Andrii Nakryiko wrote:
> On Thu, Sep 5, 2024 at 12:21 PM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>>
>>
>>
>> On 2024-09-05 2:00 p.m., Andrii Nakryiko wrote:
>>> It's incorrect to assume that LBR can/should only be used with sampling
>>> events. BPF subsystem provides bpf_get_branch_snapshot() BPF helper,
>>> which expects a properly setup and activated perf event which allows
>>> kernel to capture LBR data.
>>>
>>> For instance, retsnoop tool ([0]) makes an extensive use of this
>>> functionality and sets up perf event as follows:
>>>
>>> struct perf_event_attr attr;
>>>
>>> memset(&attr, 0, sizeof(attr));
>>> attr.size = sizeof(attr);
>>> attr.type = PERF_TYPE_HARDWARE;
>>> attr.config = PERF_COUNT_HW_CPU_CYCLES;
>>> attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
>>> attr.branch_sample_type = PERF_SAMPLE_BRANCH_KERNEL;
>>>
>>> Commit referenced in Fixes tag broke this setup by making invalid assumption
>>> that LBR is useful only for sampling events. Remove that assumption.
>>>
>>> Note, earlier we removed a similar assumption on AMD side of LBR support,
>>> see [1] for details.
>>>
>>> [0] https://github.com/anakryiko/retsnoop
>>> [1] 9794563d4d05 ("perf/x86/amd: Don't reject non-sampling events with configured LBR")
>>>
>>> Cc: stable@...r.kernel.org # 6.8+
>>> Fixes: 85846b27072d ("perf/x86: Add PERF_X86_EVENT_NEEDS_BRANCH_STACK flag")
>>> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
>>> ---
>>> arch/x86/events/intel/core.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>>> index 9e519d8a810a..f82a342b8852 100644
>>> --- a/arch/x86/events/intel/core.c
>>> +++ b/arch/x86/events/intel/core.c
>>> @@ -3972,7 +3972,7 @@ static int intel_pmu_hw_config(struct perf_event *event)
>>> x86_pmu.pebs_aliases(event);
>>> }
>>>
>>> - if (needs_branch_stack(event) && is_sampling_event(event))
>>> + if (needs_branch_stack(event))
>>> event->hw.flags |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
>>
>> To limit the LBR for a sampling event is to avoid unnecessary branch
>> stack setup for a counting event in the sample read. The above change
>> should break the sample read case.
>>
>> How about the below patch (not test)? Is it good enough for the BPF usage?
>>
>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>> index 0c9c2706d4ec..8d67cbda916b 100644
>> --- a/arch/x86/events/intel/core.c
>> +++ b/arch/x86/events/intel/core.c
>> @@ -3972,8 +3972,12 @@ static int intel_pmu_hw_config(struct perf_event
>> *event)
>> x86_pmu.pebs_aliases(event);
>> }
>>
>> - if (needs_branch_stack(event) && is_sampling_event(event))
>> - event->hw.flags |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
>> + if (needs_branch_stack(event)) {
>> + /* Avoid branch stack setup for counting events in SAMPLE READ */
>> + if (is_sampling_event(event) ||
>> + !(event->attr.sample_type & PERF_SAMPLE_READ))
>> + event->hw.flags |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
>> + }
>>
>
> I'm sure it will be fine for my use case, as I set only
> PERF_SAMPLE_BRANCH_STACK.
>
> But I'll leave it up to perf subsystem experts to decide if this
> condition makes sense, because looking at what PERF_SAMPLE_READ is:
>
> PERF_SAMPLE_READ
> Record counter values for all events in a group,
> not just the group leader.
>
> It's not clear why this would disable LBR, if specified.
It only disables the counting event with SAMPLE_READ, since LBR is only
read in the sampling event's overflow.
Thanks,
Kan
>
>> if (branch_sample_counters(event)) {
>> struct perf_event *leader, *sibling;
>>
>>
>> Thanks,
>> Kan
>>>
>>> if (branch_sample_counters(event)) {
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