Re: [Patch v5 12/18] perf/x86/intel: Always snapshot live hardware LBR state for BPF

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 10/29/2026 6:56 PM, Peter Zijlstra wrote:
> On Thu, Oct 29, 2026 at 12:39:08PM +0800, Dapeng Mi wrote:
>> __intel_pmu_snapshot_branch_stack() is intended to capture the current
>> live hardware LBR state for the BPF path. However, that snapshot is
>> currently skipped when all LBR users are PEBS events.
>>
>> BPF LBR snapshotting now uses its own dedicated LBR stack, so there is
>> no longer a risk of clobbering the shared default LBR stack. As a
>> result, always snapshot the live hardware LBR state for the BPF path.
>>
>> Fixes: c22ac2a3d4bd ("perf: Enable branch record for software events")
>> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
>> ---
>>  arch/x86/events/intel/core.c | 10 ++++++++--
>>  arch/x86/events/intel/lbr.c  |  9 ---------
>>  2 files changed, 8 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>> index 482a148916bf..110fff65bd50 100644
>> --- a/arch/x86/events/intel/core.c
>> +++ b/arch/x86/events/intel/core.c
>> @@ -4069,8 +4069,14 @@ static int intel_pmu_handle_irq(struct pt_regs *regs)
>>  
>>  	loops = 0;
>>  again:
>> -	intel_pmu_lbr_read(&cpuc->lbr_stack,
>> -			   cpuc->lbr_entries, cpuc->lbr_counters);
>> +	/*
>> +	 * No need to read the LBR stack when all LBR users are PEBS events,
>> +	 * since the PEBS handler updates it.
>> +	 */
>> +	if (cpuc->lbr_users != cpuc->lbr_pebs_users) {
>> +		intel_pmu_lbr_read(&cpuc->lbr_stack,
>> +				   cpuc->lbr_entries, cpuc->lbr_counters);
>> +	}
>>  
>>  	/*
>>  	 * Do not clear the LBR frozen status here; keep LBR frozen
>> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
>> index 63f0504f9d2a..790580f96040 100644
>> --- a/arch/x86/events/intel/lbr.c
>> +++ b/arch/x86/events/intel/lbr.c
>> @@ -1050,15 +1050,6 @@ void intel_pmu_lbr_read(struct perf_branch_stack *lbr_stack,
>>  		return;
>>  	}
>>  
>> -	/*
>> -	 * Don't read when all LBRs users are using adaptive PEBS.
>> -	 *
>> -	 * This could be smarter and actually check the event,
>> -	 * but this simple approach seems to work for now.
>> -	 */
>> -	if (cpuc->lbr_users == cpuc->lbr_pebs_users)
>> -		return;
>> -
>>  	x86_pmu.lbr_read(cpuc, lbr_stack, lbr_entries);
>>  
>>  	intel_pmu_lbr_filter(cpuc, lbr_stack, lbr_entries, lbr_counters);
> AFAICT AMD could be similarly modified, it currently unconditionally
> reads the LBR stack, irrespective of lbr_users.

Yes, LBR stack is only needed to snapshot when lbr_users > 0.

Thanks.