Re: [PATCH] perf/x86: fix wrong assumption that LBR is only useful for sampling events

["Liang, Kan" <kan.liang@linux.intel.com>]

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@vger.kernel.org # 6.8+
> Fixes: 85846b27072d ("perf/x86: Add PERF_X86_EVENT_NEEDS_BRANCH_STACK flag")
> Signed-off-by: Andrii Nakryiko <andrii@kernel.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;
+	}

 	if (branch_sample_counters(event)) {
 		struct perf_event *leader, *sibling;


Thanks,
Kan
>  
>  	if (branch_sample_counters(event)) {