Hello Namhyung,

On 6/2/23 08:03, Namhyung Kim wrote:

On Tue, May 30, 2023 at 9:27 PM Anshuman Khandual
[off-list ref] wrote:

quoted

This enables support for branch stack sampling event in ARMV8 PMU, checking
has_branch_stack() on the event inside 'struct arm_pmu' callbacks. Although
these branch stack helpers armv8pmu_branch_XXXXX() are just dummy functions
for now. While here, this also defines arm_pmu's sched_task() callback with
armv8pmu_sched_task(), which resets the branch record buffer on a sched_in.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Tested-by: James Clark <redacted>
Signed-off-by: Anshuman Khandual <redacted>
---
 arch/arm64/include/asm/perf_event.h | 33 +++++++++++++
 drivers/perf/arm_pmuv3.c            | 76 ++++++++++++++++++++---------
 2 files changed, 86 insertions(+), 23 deletions(-)

diff --git a/arch/arm64/include/asm/perf_event.h b/arch/arm64/include/asm/perf_event.h
index eb7071c9eb34..7548813783ba 100644
--- a/arch/arm64/include/asm/perf_event.h
+++ b/arch/arm64/include/asm/perf_event.h

@@ -24,4 +24,37 @@ extern unsigned long perf_misc_flags(struct pt_regs *regs);
        (regs)->pstate = PSR_MODE_EL1h; \
 }

+struct pmu_hw_events;
+struct arm_pmu;
+struct perf_event;
+
+#ifdef CONFIG_PERF_EVENTS
+static inline bool has_branch_stack(struct perf_event *event);
+
+static inline void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event)
+{
+       WARN_ON_ONCE(!has_branch_stack(event));
+}
+
+static inline bool armv8pmu_branch_valid(struct perf_event *event)
+{
+       WARN_ON_ONCE(!has_branch_stack(event));
+       return false;
+}
+
+static inline void armv8pmu_branch_enable(struct perf_event *event)
+{
+       WARN_ON_ONCE(!has_branch_stack(event));
+}
+
+static inline void armv8pmu_branch_disable(struct perf_event *event)
+{
+       WARN_ON_ONCE(!has_branch_stack(event));
+}
+
+static inline void armv8pmu_branch_probe(struct arm_pmu *arm_pmu) { }
+static inline void armv8pmu_branch_reset(void) { }
+static inline int armv8pmu_private_alloc(struct arm_pmu *arm_pmu) { return 0; }
+static inline void armv8pmu_private_free(struct arm_pmu *arm_pmu) { }
+#endif
 #endif

diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
index c98e4039386d..86d803ff1ae3 100644
--- a/drivers/perf/arm_pmuv3.c
+++ b/drivers/perf/arm_pmuv3.c

@@ -705,38 +705,21 @@ static void armv8pmu_enable_event(struct perf_event *event)
         * Enable counter and interrupt, and set the counter to count
         * the event that we're interested in.
         */
-
-       /*
-        * Disable counter
-        */
        armv8pmu_disable_event_counter(event);
-
-       /*
-        * Set event.
-        */
        armv8pmu_write_event_type(event);
-
-       /*
-        * Enable interrupt for this counter
-        */
        armv8pmu_enable_event_irq(event);
-
-       /*
-        * Enable counter
-        */
        armv8pmu_enable_event_counter(event);
+
+       if (has_branch_stack(event))
+               armv8pmu_branch_enable(event);
 }

 static void armv8pmu_disable_event(struct perf_event *event)
 {
-       /*
-        * Disable counter
-        */
-       armv8pmu_disable_event_counter(event);
+       if (has_branch_stack(event))
+               armv8pmu_branch_disable(event);

-       /*
-        * Disable interrupt for this counter
-        */
+       armv8pmu_disable_event_counter(event);
        armv8pmu_disable_event_irq(event);
 }

@@ -814,6 +797,11 @@ static irqreturn_t armv8pmu_handle_irq(struct arm_pmu *cpu_pmu)
                if (!armpmu_event_set_period(event))
                        continue;

+               if (has_branch_stack(event) && !WARN_ON(!cpuc->branches)) {
+                       armv8pmu_branch_read(cpuc, event);
+                       perf_sample_save_brstack(&data, event, &cpuc->branches->branch_stack);
+               }
+
                /*
                 * Perf event overflow will queue the processing of the event as
                 * an irq_work which will be taken care of in the handling of

@@ -912,6 +900,14 @@ static int armv8pmu_user_event_idx(struct perf_event *event)
        return event->hw.idx;
 }

+static void armv8pmu_sched_task(struct perf_event_pmu_context *pmu_ctx, bool sched_in)
+{
+       struct arm_pmu *armpmu = to_arm_pmu(pmu_ctx->pmu);
+
+       if (sched_in && arm_pmu_branch_stack_supported(armpmu))
+               armv8pmu_branch_reset();
+}
+
 /*
  * Add an event filter to a given event.
  */

@@ -982,6 +978,9 @@ static void armv8pmu_reset(void *info)
                pmcr |= ARMV8_PMU_PMCR_LP;

        armv8pmu_pmcr_write(pmcr);
+
+       if (arm_pmu_branch_stack_supported(cpu_pmu))
+               armv8pmu_branch_reset();
 }

 static int __armv8_pmuv3_map_event_id(struct arm_pmu *armpmu,

@@ -1019,6 +1018,9 @@ static int __armv8_pmuv3_map_event(struct perf_event *event,

        hw_event_id = __armv8_pmuv3_map_event_id(armpmu, event);

+       if (has_branch_stack(event) && !armv8pmu_branch_valid(event))
+               return -EOPNOTSUPP;
+
        /*
         * CHAIN events only work when paired with an adjacent counter, and it
         * never makes sense for a user to open one in isolation, as they'll be

@@ -1135,6 +1137,21 @@ static void __armv8pmu_probe_pmu(void *info)
                cpu_pmu->reg_pmmir = read_pmmir();
        else
                cpu_pmu->reg_pmmir = 0;
+       armv8pmu_branch_probe(cpu_pmu);
+}
+
+static int branch_records_alloc(struct arm_pmu *armpmu)
+{
+       struct pmu_hw_events *events;
+       int cpu;
+
+       for_each_possible_cpu(cpu) {
+               events = per_cpu_ptr(armpmu->hw_events, cpu);
+               events->branches = kzalloc(sizeof(struct branch_records), GFP_KERNEL);
+               if (!events->branches)
+                       return -ENOMEM;
+       }
+       return 0;
 }

 static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)

@@ -1145,12 +1162,24 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
        };
        int ret;

+       ret = armv8pmu_private_alloc(cpu_pmu);
+       if (ret)
+               return ret;

Wouldn't it be better to move it under the if statement below
if it's only needed for branch stack?

armv8pmu_private_alloc() allocates arm_pmu's private structure which stores
the BRBE HW attributes during armv8pmu_branch_probe(), called from this SMP
callback __armv8pmu_probe_pmu(). Hence without the structure being allocated
and assigned, following smp_call_function_any() cannot execute successfully.

armv8pmu_private_alloc()
	{
		......
		Allocates arm_pmu->private as single 'struct brbe_hw_attr'
		Allocates arm_pmu->pmu.task_ctx_cache
		......
	}

__armv8pmu_probe_pmu()
	armv8pmu_branch_probe()
		brbe_attributes_probe()
		{
			......
			brbe_attr->brbe_version = brbe;
			brbe_attr->brbe_format = brbe_get_format(brbidr);
        		brbe_attr->brbe_cc = brbe_get_cc_bits(brbidr);
        		brbe_attr->brbe_nr = brbe_get_numrec(brbidr);
			......
		}

armv8pmu_private_alloc() cannot be moved inside armv8pmu_branch_probe(),
because there cannot be any allocation while being in a SMP call context.

quoted

+
        ret = smp_call_function_any(&cpu_pmu->supported_cpus,
                                    __armv8pmu_probe_pmu,
                                    &probe, 1);
        if (ret)
                return ret;

Otherwise you might need to free it here.

quoted

+       if (arm_pmu_branch_stack_supported(cpu_pmu)) {
+               ret = branch_records_alloc(cpu_pmu);
+               if (ret)
+                       return ret;

And here too.

Not freeing the arm_pmu's private data, might not be a problem in cases
where either pmu does not support BRBE or pmu probe itself fails. But for
completeness, will change as following.

diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
index 9725a53d6799..fdbe52913cc7 100644
--- a/drivers/perf/arm_pmuv3.c
+++ b/drivers/perf/arm_pmuv3.c

@@ -1198,13 +1198,17 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
        ret = smp_call_function_any(&cpu_pmu->supported_cpus,
                                    __armv8pmu_probe_pmu,
                                    &probe, 1);
-       if (ret)
+       if (ret) {
+               armv8pmu_private_free(cpu_pmu);
                return ret;
+       }
 
        if (arm_pmu_branch_stack_supported(cpu_pmu)) {
                ret = branch_records_alloc(cpu_pmu);
-               if (ret)
+               if (ret) {
+                       armv8pmu_private_free(cpu_pmu);
                        return ret;
+               }
        } else {
                armv8pmu_private_free(cpu_pmu);
        }

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