[PATCH 2/5] x86, perf: Add option to disable reading branch flags/cycles

[Andi Kleen]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

With LBRv5 reading the extra LBR flags like mispredict, TSX, cycles
is not free anymore, as it has moved to a separate MSR.

For callstack mode we don't need any of this information; so we can
avoid the unnecessary MSR read. Add flags to the perf interface
where perf record can request not collecting this information.
I added sample_type flags for CYCLES and FLAGS. It's a bit unusual for
sample_types to be negative (disable), not positive (enable), but
since the legacy ABI reported the flags we need some form of explicit
disabling to avoid breaking the ABI. In theory it would be possible
to make CYCLES opt-in (as it's not deployed yet), but I also made it
opt-out to be symmetric to FLAGS.

After we have the flags the x86 perf code can keep track if any
users need the flags. If noone needs it the information is not
collected.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/perf_event.h           |  2 ++
 arch/x86/kernel/cpu/perf_event_intel_lbr.c | 49 ++++++++++++++++++++++--------
 include/uapi/linux/perf_event.h            |  2 ++
 3 files changed, 40 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.h b/arch/x86/kernel/cpu/perf_event.h
index 2860b89..c83bf07 100644
--- a/arch/x86/kernel/cpu/perf_event.h
+++ b/arch/x86/kernel/cpu/perf_event.h
@@ -201,6 +201,8 @@ struct cpu_hw_events {
 	 * Intel LBR bits
 	 */
 	int				lbr_users;
+	int				lbr_flags_users;
+	int				lbr_cycles_users;
 	void				*lbr_context;
 	struct perf_branch_stack	lbr_stack;
 	struct perf_branch_entry	lbr_entries[MAX_LBR_ENTRIES];
diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
index 1fd8b5a..5de2048 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
@@ -340,6 +340,10 @@ void intel_pmu_lbr_enable(struct perf_event *event)
 	}
 
 	cpuc->lbr_users++;
+	if (!(event->attr.sample_type & PERF_SAMPLE_BRANCH_NO_FLAGS))
+		cpuc->lbr_flags_users++;
+	if (!(event->attr.sample_type & PERF_SAMPLE_BRANCH_NO_CYCLES))
+		cpuc->lbr_cycles_users++;
 	perf_sched_cb_inc(event->ctx->pmu);
 }
 
@@ -358,7 +362,14 @@ void intel_pmu_lbr_disable(struct perf_event *event)
 	}
 
 	cpuc->lbr_users--;
-	WARN_ON_ONCE(cpuc->lbr_users < 0);
+	if (!(event->attr.sample_type & PERF_SAMPLE_BRANCH_NO_FLAGS))
+		cpuc->lbr_flags_users--;
+	if (!(event->attr.sample_type & PERF_SAMPLE_BRANCH_NO_CYCLES))
+		cpuc->lbr_cycles_users--;
+
+	WARN_ON_ONCE(cpuc->lbr_users < 0 ||
+		     cpuc->lbr_flags_users < 0 ||
+		     cpuc->lbr_cycles_users < 0);
 	perf_sched_cb_dec(event->ctx->pmu);
 
 	if (cpuc->enabled && !cpuc->lbr_users) {
@@ -416,7 +427,9 @@ static void intel_pmu_lbr_read_32(struct cpu_hw_events *cpuc)
  * is the same as the linear address, allowing us to merge the LIP and EIP
  * LBR formats.
  */
-static void intel_pmu_lbr_read_64(struct cpu_hw_events *cpuc)
+static void intel_pmu_lbr_read_64(struct cpu_hw_events *cpuc,
+				  bool need_flags,
+				  bool need_cycles)
 {
 	unsigned long mask = x86_pmu.lbr_nr - 1;
 	int lbr_format = x86_pmu.intel_cap.lbr_format;
@@ -434,24 +447,32 @@ static void intel_pmu_lbr_read_64(struct cpu_hw_events *cpuc)
 		rdmsrl(x86_pmu.lbr_from + lbr_idx, from);
 		rdmsrl(x86_pmu.lbr_to   + lbr_idx, to);
 
-		if (lbr_format == LBR_FORMAT_INFO) {
+		if (lbr_format == LBR_FORMAT_INFO &&
+		    (need_flags || need_cycles)) {
 			u64 info;
 
 			rdmsrl(MSR_LBR_INFO_0 + lbr_idx, info);
-			mis = !!(info & LBR_INFO_MISPRED);
-			pred = !mis;
-			in_tx = !!(info & LBR_INFO_IN_TX);
-			abort = !!(info & LBR_INFO_ABORT);
-			cycles = (info & LBR_INFO_CYCLES);
+			if (need_flags) {
+				mis = !!(info & LBR_INFO_MISPRED);
+				pred = !mis;
+				in_tx = !!(info & LBR_INFO_IN_TX);
+				abort = !!(info & LBR_INFO_ABORT);
+			}
+			if (need_cycles)
+				cycles = (info & LBR_INFO_CYCLES);
 		}
 		if (lbr_flags & LBR_EIP_FLAGS) {
-			mis = !!(from & LBR_FROM_FLAG_MISPRED);
-			pred = !mis;
+			if (need_flags) {
+				mis = !!(from & LBR_FROM_FLAG_MISPRED);
+				pred = !mis;
+			}
 			skip = 1;
 		}
 		if (lbr_flags & LBR_TSX) {
-			in_tx = !!(from & LBR_FROM_FLAG_IN_TX);
-			abort = !!(from & LBR_FROM_FLAG_ABORT);
+			if (need_flags) {
+				in_tx = !!(from & LBR_FROM_FLAG_IN_TX);
+				abort = !!(from & LBR_FROM_FLAG_ABORT);
+			}
 			skip = 3;
 		}
 		from = (u64)((((s64)from) << skip) >> skip);
@@ -490,7 +511,9 @@ void intel_pmu_lbr_read(void)
 	if (x86_pmu.intel_cap.lbr_format == LBR_FORMAT_32)
 		intel_pmu_lbr_read_32(cpuc);
 	else
-		intel_pmu_lbr_read_64(cpuc);
+		intel_pmu_lbr_read_64(cpuc,
+				      cpuc->lbr_flags_users > 0,
+				      cpuc->lbr_cycles_users > 0);
 
 	intel_pmu_lbr_filter(cpuc);
 }
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 1b8bd4a..8dd5765 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -138,6 +138,8 @@ enum perf_event_sample_format {
 	PERF_SAMPLE_IDENTIFIER			= 1U << 16,
 	PERF_SAMPLE_TRANSACTION			= 1U << 17,
 	PERF_SAMPLE_REGS_INTR			= 1U << 18,
+	PERF_SAMPLE_BRANCH_NO_FLAGS		= 1U << 19,
+	PERF_SAMPLE_BRANCH_NO_CYCLES		= 1U << 20,
 
 	PERF_SAMPLE_MAX = 1U << 19,		/* non-ABI */
 };
-- 
2.1.0

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