[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 branch_sample_type flags for CYCLES and FLAGS. It's a bit unusual for
branch_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.

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.

This cuts down the cost of LBR callstack on Skylake significantly.
Profiling a kernel build with LBR call stack the average run time of
the PMI handler drops by 43%

v2: Rework based on Peter's feedback.
Moved the NO_FLAGS/NO_CYCLES bits to branch_sample_type.
Remove extra checks for no info, so that the flags are filled in if
there is no lbr_info or if only one is set to reduce number of branches.
Instead of counters the information about skipping LBR_INFO is now
kept as a extra register bit that is filtered out before writing
the register to hardware.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/perf_event_intel_lbr.c | 19 +++++++++++++++++--
 include/uapi/linux/perf_event.h            |  6 ++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
index 0e4ea00..60e71b7 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
@@ -42,6 +42,13 @@ static enum {
 #define LBR_FAR_BIT		8 /* do not capture far branches */
 #define LBR_CALL_STACK_BIT	9 /* enable call stack */
 
+/*
+ * Following bit only exists in Linux; we mask it out before writing it to
+ * the actual MSR. But it helps the constraint perf code to understand
+ * that this is a separate configuration.
+ */
+#define LBR_NO_INFO_BIT	       63 /* don't read LBR_INFO. */
+
 #define LBR_KERNEL	(1 << LBR_KERNEL_BIT)
 #define LBR_USER	(1 << LBR_USER_BIT)
 #define LBR_JCC		(1 << LBR_JCC_BIT)
@@ -52,6 +59,7 @@ static enum {
 #define LBR_IND_JMP	(1 << LBR_IND_JMP_BIT)
 #define LBR_FAR		(1 << LBR_FAR_BIT)
 #define LBR_CALL_STACK	(1 << LBR_CALL_STACK_BIT)
+#define LBR_NO_INFO	(1ULL << LBR_NO_INFO_BIT)
 
 #define LBR_PLM (LBR_KERNEL | LBR_USER)
 
@@ -152,7 +160,7 @@ static void __intel_pmu_lbr_enable(bool pmi)
 	 * did not change.
 	 */
 	if (cpuc->lbr_sel)
-		lbr_select = cpuc->lbr_sel->config;
+		lbr_select = cpuc->lbr_sel->config & x86_pmu.lbr_sel_mask;
 	if (!pmi)
 		wrmsrl(MSR_LBR_SELECT, lbr_select);
 
@@ -422,6 +430,7 @@ static void intel_pmu_lbr_read_32(struct cpu_hw_events *cpuc)
  */
 static void intel_pmu_lbr_read_64(struct cpu_hw_events *cpuc)
 {
+	bool need_info = !(cpuc->lbr_sel->config & LBR_NO_INFO);
 	unsigned long mask = x86_pmu.lbr_nr - 1;
 	int lbr_format = x86_pmu.intel_cap.lbr_format;
 	u64 tos = intel_pmu_lbr_tos();
@@ -442,7 +451,7 @@ 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_info) {
 			u64 info;
 
 			rdmsrl(MSR_LBR_INFO_0 + lbr_idx, info);
@@ -588,6 +597,7 @@ static int intel_pmu_setup_hw_lbr_filter(struct perf_event *event)
 		if (v != LBR_IGN)
 			mask |= v;
 	}
+
 	reg = &event->hw.branch_reg;
 	reg->idx = EXTRA_REG_LBR;
 
@@ -598,6 +608,11 @@ static int intel_pmu_setup_hw_lbr_filter(struct perf_event *event)
 	 */
 	reg->config = mask ^ x86_pmu.lbr_sel_mask;
 
+	if ((br_type & PERF_SAMPLE_BRANCH_NO_CYCLES) &&
+	    (br_type & PERF_SAMPLE_BRANCH_NO_FLAGS) &&
+	    (x86_pmu.intel_cap.lbr_format == LBR_FORMAT_INFO))
+		reg->config |= LBR_NO_INFO;
+
 	return 0;
 }
 
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 2881145..a8ffc76 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -169,6 +169,9 @@ enum perf_branch_sample_type_shift {
 	PERF_SAMPLE_BRANCH_CALL_STACK_SHIFT	= 11, /* call/ret stack */
 	PERF_SAMPLE_BRANCH_IND_JUMP_SHIFT	= 12, /* indirect jumps */
 
+	PERF_SAMPLE_BRANCH_NO_FLAGS_SHIFT	= 13, /* no flags */
+	PERF_SAMPLE_BRANCH_NO_CYCLES_SHIFT	= 14, /* no cycles */
+
 	PERF_SAMPLE_BRANCH_MAX_SHIFT		/* non-ABI */
 };
 
@@ -189,6 +192,9 @@ enum perf_branch_sample_type {
 	PERF_SAMPLE_BRANCH_CALL_STACK	= 1U << PERF_SAMPLE_BRANCH_CALL_STACK_SHIFT,
 	PERF_SAMPLE_BRANCH_IND_JUMP	= 1U << PERF_SAMPLE_BRANCH_IND_JUMP_SHIFT,
 
+	PERF_SAMPLE_BRANCH_NO_FLAGS	= 1U << PERF_SAMPLE_BRANCH_NO_FLAGS_SHIFT,
+	PERF_SAMPLE_BRANCH_NO_CYCLES	= 1U << PERF_SAMPLE_BRANCH_NO_CYCLES_SHIFT,
+
 	PERF_SAMPLE_BRANCH_MAX		= 1U << PERF_SAMPLE_BRANCH_MAX_SHIFT,
 };
 
-- 
2.4.3

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/