[PATCH v3 13/14] perf, x86: enable LBR callstack when recording callchain

[Yan, Zheng]

Try enabling the LBR callstack facility if user requests recording
user callchain. Also adds a cpu pmu attribute to enable/disable this
feature. This feature is disabled by default because it may contend
for the LBR with other events that explicitly require branch stack.

Note: this feature only affects how to get user callchain. The kernel
callchain is always got by frame pointers.

Signed-off-by: Yan, Zheng <zheng.z.yan@xxxxxxxxx>
---
 arch/x86/kernel/cpu/perf_event.c | 99 ++++++++++++++++++++++++++++------------
 arch/x86/kernel/cpu/perf_event.h |  1 +
 2 files changed, 71 insertions(+), 29 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 0d0fe2f3..837a0b1 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -399,37 +399,49 @@ int x86_pmu_hw_config(struct perf_event *event)
 
 		if (event->attr.precise_ip > precise)
 			return -EOPNOTSUPP;
+	}
+	/*
+	 * check that PEBS LBR correction does not conflict with
+	 * whatever the user is asking with attr->branch_sample_type
+	 */
+	if (event->attr.precise_ip > 1 && x86_pmu.intel_cap.pebs_format < 2) {
+		u64 *br_type = &event->attr.branch_sample_type;
+
+		if (has_branch_stack(event)) {
+			if (!precise_br_compat(event))
+				return -EOPNOTSUPP;
+
+			/* branch_sample_type is compatible */
+
+		} else {
+			/*
+			 * user did not specify  branch_sample_type
+			 *
+			 * For PEBS fixups, we capture all
+			 * the branches at the priv level of the
+			 * event.
+			 */
+			*br_type = PERF_SAMPLE_BRANCH_ANY;
+
+			if (!event->attr.exclude_user)
+				*br_type |= PERF_SAMPLE_BRANCH_USER;
+
+			if (!event->attr.exclude_kernel)
+				*br_type |= PERF_SAMPLE_BRANCH_KERNEL;
+		}
+	} else if ((event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) &&
+		   !has_branch_stack(event) &&
+		   x86_pmu.attr_lbr_callstack &&
+		   !event->attr.exclude_user &&
+		   (event->attach_state & PERF_ATTACH_TASK)) {
 		/*
-		 * check that PEBS LBR correction does not conflict with
-		 * whatever the user is asking with attr->branch_sample_type
+		 * user did not specify branch_sample_type,
+		 * try using the LBR call stack facility to
+		 * record call chains of user program.
 		 */
-		if (event->attr.precise_ip > 1 &&
-		    x86_pmu.intel_cap.pebs_format < 2) {
-			u64 *br_type = &event->attr.branch_sample_type;
-
-			if (has_branch_stack(event)) {
-				if (!precise_br_compat(event))
-					return -EOPNOTSUPP;
-
-				/* branch_sample_type is compatible */
-
-			} else {
-				/*
-				 * user did not specify  branch_sample_type
-				 *
-				 * For PEBS fixups, we capture all
-				 * the branches at the priv level of the
-				 * event.
-				 */
-				*br_type = PERF_SAMPLE_BRANCH_ANY;
-
-				if (!event->attr.exclude_user)
-					*br_type |= PERF_SAMPLE_BRANCH_USER;
-
-				if (!event->attr.exclude_kernel)
-					*br_type |= PERF_SAMPLE_BRANCH_KERNEL;
-			}
-		}
+		event->attr.branch_sample_type =
+			PERF_SAMPLE_BRANCH_USER |
+			PERF_SAMPLE_BRANCH_CALL_STACK;
 	}
 
 	/*
@@ -1832,10 +1844,39 @@ static ssize_t set_attr_rdpmc(struct device *cdev,
 	return count;
 }
 
+static ssize_t get_attr_lbr_callstack(struct device *cdev,
+				      struct device_attribute *attr, char *buf)
+{
+	return snprintf(buf, 40, "%d\n", x86_pmu.attr_lbr_callstack);
+}
+
+static ssize_t set_attr_lbr_callstack(struct device *cdev,
+				      struct device_attribute *attr,
+				      const char *buf, size_t count)
+{
+	unsigned long val;
+	ssize_t ret;
+
+	ret = kstrtoul(buf, 0, &val);
+	if (ret)
+		return ret;
+
+	if (!!val != !!x86_pmu.attr_lbr_callstack) {
+		if (val && !x86_pmu_has_lbr_callstack())
+			return -EOPNOTSUPP;
+		x86_pmu.attr_lbr_callstack = !!val;
+	}
+	return count;
+}
+
 static DEVICE_ATTR(rdpmc, S_IRUSR | S_IWUSR, get_attr_rdpmc, set_attr_rdpmc);
+static DEVICE_ATTR(lbr_callstack, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH,
+		   get_attr_lbr_callstack, set_attr_lbr_callstack);
+
 
 static struct attribute *x86_pmu_attrs[] = {
 	&dev_attr_rdpmc.attr,
+	&dev_attr_lbr_callstack.attr,
 	NULL,
 };
 
diff --git a/arch/x86/kernel/cpu/perf_event.h b/arch/x86/kernel/cpu/perf_event.h
index 61b1e9c..3798e0d 100644
--- a/arch/x86/kernel/cpu/perf_event.h
+++ b/arch/x86/kernel/cpu/perf_event.h
@@ -410,6 +410,7 @@ struct x86_pmu {
 	 */
 	int		attr_rdpmc_broken;
 	int		attr_rdpmc;
+	int		attr_lbr_callstack;
 	struct attribute **format_attrs;
 	struct attribute **event_attrs;
 
-- 
1.8.5.3

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