Re: [PATCH] perf/x86/intel: Fix lbr event can placed into non lbr group

[Liang, Kan]

On 2025-04-12 5:14 a.m., Luo Gengkun wrote:
> The following perf command can trigger a warning on
> intel_pmu_lbr_counters_reorder.
> 
>  # perf record -e "{cpu-clock,cycles/call-graph="lbr"/}" -- sleep 1
> 
> The reason is that a lbr event are placed in non lbr group. And the
> previous implememtation cannot force the leader to be a lbr event in this
> case. 

Perf should only force the LBR leader for the branch counters case, so
perf only needs to reset the LBRs for the leader.
I don't think the leader restriction should be applied to other cases.

> And is_branch_counters_group will check if the group_leader supports
> BRANCH_COUNTERS.
> So if a software event becomes a group_leader, which
> hw.flags is -1, this check will alway pass.

I think the default flags for all events is 0. Can you point me to where
it is changed to -1?

Thanks,
Kan>
> To fix this problem, using has_branch_stack to judge if leader is lbr
> event.
> 
> Fixes: 33744916196b ("perf/x86/intel: Support branch counters logging")
> Signed-off-by: Luo Gengkun <luogengkun@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/events/intel/core.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 09d2d66c9f21..c6b394019e54 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4114,6 +4114,13 @@ static int intel_pmu_hw_config(struct perf_event *event)
>  			event->hw.flags |= PERF_X86_EVENT_NEEDS_BRANCH_STACK;
>  	}
>  
> +	/*
> +	 * Force the leader to be a LBR event. So LBRs can be reset
> +	 * with the leader event. See intel_pmu_lbr_del() for details.
> +	 */
> +	if (has_branch_stack(event) && !has_branch_stack(event->group_leader))
> +		return -EINVAL;
> +
>  	if (branch_sample_counters(event)) {
>  		struct perf_event *leader, *sibling;
>  		int num = 0;
> @@ -4157,13 +4164,6 @@ static int intel_pmu_hw_config(struct perf_event *event)
>  			  ~(PERF_SAMPLE_BRANCH_PLM_ALL |
>  			    PERF_SAMPLE_BRANCH_COUNTERS)))
>  			event->hw.flags  &= ~PERF_X86_EVENT_NEEDS_BRANCH_STACK;
> -
> -		/*
> -		 * Force the leader to be a LBR event. So LBRs can be reset
> -		 * with the leader event. See intel_pmu_lbr_del() for details.
> -		 */
> -		if (!intel_pmu_needs_branch_stack(leader))
> -			return -EINVAL;
>  	}
>  
>  	if (intel_pmu_needs_branch_stack(event)) {