Re: [PATCH V4 1/2] perf/x86/intel/ds: Flush the PEBS buffer in PEBS enable

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

On 2023-04-26 9:18 a.m., Peter Zijlstra wrote:
> On Fri, Apr 21, 2023 at 11:45:28AM -0700, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> Several similar kernel warnings can be triggered,
>>
>>   [56605.607840] CPU0 PEBS record size 0, expected 32, config 0
>>   cpuc->record_size=208
>>
>> when the below commands are running in parallel for a while on SPR.
>>
>>   while true; do perf record --no-buildid -a --intr-regs=AX -e
>>   cpu/event=0xd0,umask=0x81/pp -c 10003 -o /dev/null ./triad; done &
>>
>>   while true; do perf record -o /tmp/out -W -d -e
>>   '{ld_blocks.store_forward:period=1000000,
>>   MEM_TRANS_RETIRED.LOAD_LATENCY:u:precise=2:ldlat=4}'
>>   -c 1037 ./triad; done
>>   *The triad program is just the generation of loads/stores.
>>
>> The warnings are triggered when an unexpected PEBS record (with a
>> different config and size) is found.
>>
>> A system-wide PEBS event with the large PEBS config may be enabled
>> during a context switch. Some PEBS records for the system-wide PEBS may
>> be generated while the old task is sched out but the new one hasn't been
>> sched in yet. When the new task is sched in, the cpuc->pebs_record_size
>> may be updated for the per-task PEBS events. So the existing system-wide
>> PEBS records have a different size from the later PEBS records.
>>
>> The PEBS buffer should be flushed right before the hardware is
>> reprogrammed. The new size and threshold should be updated after the old
>> buffer has been flushed.
>>
>> Reported-by: Stephane Eranian <eranian@google.com>
>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
>> ---
> 
> So I find it much easier to read the whole thing when collapsed.
> Something like the below; that ok with you?

Yes, the changes look good to me.

Thanks,
Kan
> 
> ---
>  arch/x86/events/intel/ds.c        | 56 ++++++++++++++++++++++-----------------
>  arch/x86/include/asm/perf_event.h |  3 +++
>  2 files changed, 35 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index a2e566e53076..df88576d6b2a 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -1229,12 +1229,14 @@ pebs_update_state(bool needed_cb, struct cpu_hw_events *cpuc,
>  		  struct perf_event *event, bool add)
>  {
>  	struct pmu *pmu = event->pmu;
> +
>  	/*
>  	 * Make sure we get updated with the first PEBS
>  	 * event. It will trigger also during removal, but
>  	 * that does not hurt:
>  	 */
> -	bool update = cpuc->n_pebs == 1;
> +	if (cpuc->n_pebs == 1)
> +		cpuc->pebs_data_cfg = PEBS_UPDATE_DS_SW;
>  
>  	if (needed_cb != pebs_needs_sched_cb(cpuc)) {
>  		if (!needed_cb)
> @@ -1242,7 +1244,7 @@ pebs_update_state(bool needed_cb, struct cpu_hw_events *cpuc,
>  		else
>  			perf_sched_cb_dec(pmu);
>  
> -		update = true;
> +		cpuc->pebs_data_cfg |= PEBS_UPDATE_DS_SW;
>  	}
>  
>  	/*
> @@ -1252,24 +1254,13 @@ pebs_update_state(bool needed_cb, struct cpu_hw_events *cpuc,
>  	if (x86_pmu.intel_cap.pebs_baseline && add) {
>  		u64 pebs_data_cfg;
>  
> -		/* Clear pebs_data_cfg and pebs_record_size for first PEBS. */
> -		if (cpuc->n_pebs == 1) {
> -			cpuc->pebs_data_cfg = 0;
> -			cpuc->pebs_record_size = sizeof(struct pebs_basic);
> -		}
> -
>  		pebs_data_cfg = pebs_update_adaptive_cfg(event);
> -
> -		/* Update pebs_record_size if new event requires more data. */
> -		if (pebs_data_cfg & ~cpuc->pebs_data_cfg) {
> -			cpuc->pebs_data_cfg |= pebs_data_cfg;
> -			adaptive_pebs_record_size_update();
> -			update = true;
> -		}
> +		/*
> +		 * Be sure to update the thresholds when we change the record.
> +		 */
> +		if (pebs_data_cfg & ~cpuc->pebs_data_cfg)
> +			cpuc->pebs_data_cfg |= pebs_data_cfg | PEBS_UPDATE_DS_SW;
>  	}
> -
> -	if (update)
> -		pebs_update_threshold(cpuc);
>  }
>  
>  void intel_pmu_pebs_add(struct perf_event *event)
> @@ -1326,9 +1317,17 @@ static void intel_pmu_pebs_via_pt_enable(struct perf_event *event)
>  	wrmsrl(base + idx, value);
>  }
>  
> +static inline void intel_pmu_drain_large_pebs(struct cpu_hw_events *cpuc)
> +{
> +	if (cpuc->n_pebs == cpuc->n_large_pebs &&
> +	    cpuc->n_pebs != cpuc->n_pebs_via_pt)
> +		intel_pmu_drain_pebs_buffer();
> +}
> +
>  void intel_pmu_pebs_enable(struct perf_event *event)
>  {
>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	u64 pebs_data_cfg = cpuc->pebs_data_cfg & ~PEBS_UPDATE_DS_SW;
>  	struct hw_perf_event *hwc = &event->hw;
>  	struct debug_store *ds = cpuc->ds;
>  	unsigned int idx = hwc->idx;
> @@ -1344,11 +1343,22 @@ void intel_pmu_pebs_enable(struct perf_event *event)
>  
>  	if (x86_pmu.intel_cap.pebs_baseline) {
>  		hwc->config |= ICL_EVENTSEL_ADAPTIVE;
> -		if (cpuc->pebs_data_cfg != cpuc->active_pebs_data_cfg) {
> -			wrmsrl(MSR_PEBS_DATA_CFG, cpuc->pebs_data_cfg);
> -			cpuc->active_pebs_data_cfg = cpuc->pebs_data_cfg;
> +		if (pebs_data_cfg != cpuc->active_pebs_data_cfg) {
> +			/*
> +			 * drain_pebs() assumes uniform record size;
> +			 * hence we need to drain when changing said
> +			 * size.
> +			 */
> +			intel_pmu_drain_large_pebs(cpuc);
> +			adaptive_pebs_record_size_update();
> +			wrmsrl(MSR_PEBS_DATA_CFG, pebs_data_cfg);
> +			cpuc->active_pebs_data_cfg = pebs_data_cfg;
>  		}
>  	}
> +	if (cpuc->pebs_data_cfg & PEBS_UPDATE_DS_SW) {
> +		cpuc->pebs_data_cfg = pebs_data_cfg;
> +		pebs_update_threshold(cpuc);
> +	}
>  
>  	if (idx >= INTEL_PMC_IDX_FIXED) {
>  		if (x86_pmu.intel_cap.pebs_format < 5)
> @@ -1391,9 +1401,7 @@ void intel_pmu_pebs_disable(struct perf_event *event)
>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>  	struct hw_perf_event *hwc = &event->hw;
>  
> -	if (cpuc->n_pebs == cpuc->n_large_pebs &&
> -	    cpuc->n_pebs != cpuc->n_pebs_via_pt)
> -		intel_pmu_drain_pebs_buffer();
> +	intel_pmu_drain_large_pebs(cpuc);
>  
>  	cpuc->pebs_enabled &= ~(1ULL << hwc->idx);
>  
> diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> index 8fc15ed5e60b..abf09882f58b 100644
> --- a/arch/x86/include/asm/perf_event.h
> +++ b/arch/x86/include/asm/perf_event.h
> @@ -121,6 +121,9 @@
>  #define PEBS_DATACFG_LBRS	BIT_ULL(3)
>  #define PEBS_DATACFG_LBR_SHIFT	24
>  
> +/* Steal the highest bit of pebs_data_cfg for SW usage */
> +#define PEBS_UPDATE_DS_SW	BIT_ULL(63)
> +
>  /*
>   * Intel "Architectural Performance Monitoring" CPUID
>   * detection/enumeration details: