On 2024-09-05 4:33 p.m., Andrii Nakryiko wrote: > On Thu, Sep 5, 2024 at 1:29 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote: >> >> >> >> On 2024-09-05 4:22 p.m., Andrii Nakryiko wrote: >>> On Thu, Sep 5, 2024 at 12:21 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx # 6.8+ >>>>> Fixes: 85846b27072d ("perf/x86: Add PERF_X86_EVENT_NEEDS_BRANCH_STACK flag") >>>>> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> >>>>> --- >>>>> 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. >> > > Ok, sounds good! Would you like to send a proper patch with your > proposed changes? The patch has been posted. Please give it a try. https://lore.kernel.org/lkml/20240909155848.326640-1-kan.liang@xxxxxxxxxxxxxxx/ Thanks, Kan > >> Thanks, >> Kan >>> >>>> if (branch_sample_counters(event)) { >>>> struct perf_event *leader, *sibling; >>>> >>>> >>>> Thanks, >>>> Kan >>>>> >>>>> if (branch_sample_counters(event)) { >
