Subject: [PATCH v26a 1/2] instrumentation: Show additional TSC clock source info in pg_test_timing

From: Lukas Fittl <[email-masked]>
Date: Wed, 8 Apr 2026 10:11:56 -0700

In some cases its needed to understand whether TSC frequency data was
sourced from CPUID, and which of the registers. This shows this debug
information at the end of pg_test_timing, through use of a new
pg_timing_tsc_clock_source_info function, replacing the previous
pg_tsc_calibrate_frequency export that was only needed for debug info.

Author: Lukas Fittl <[email-masked]>
Suggested-by: Andres Freund <[email-masked]>
Discussion: https://www.postgresql.org/message-id/flat/jr4hk2sxhqcfpb67ftz5g4vw33nm67cgf7go3wwmqsafu5aclq%405m67ukuhyszz#2fdfd95b6a4a74410196999818e16cfc
---
 src/include/port/pg_cpu.h               |  2 +-
 src/include/portability/instr_time.h    | 11 ++++++-
 src/common/instr_time.c                 | 44 +++++++++++++++++++++++--
 src/port/pg_cpu_x86.c                   | 23 +++++++++++--
 src/bin/pg_test_timing/pg_test_timing.c | 12 ++++---
 src/tools/pgindent/typedefs.list        |  1 +
 6 files changed, 81 insertions(+), 12 deletions(-)

diff --git a/src/include/port/pg_cpu.h b/src/include/port/pg_cpu.h
index a5d42f1b68d..db4cd62f7ef 100644
--- a/src/include/port/pg_cpu.h
+++ b/src/include/port/pg_cpu.h
@@ -56,7 +56,7 @@ x86_feature_available(X86FeatureId feature)
 	return X86Features[feature];
 }
 
-extern uint32 x86_tsc_frequency_khz(void);
+extern uint32 x86_tsc_frequency_khz(char *source, size_t source_len);
 
 #endif							/* defined(USE_SSE2) || defined(__i386__) */
 
diff --git a/src/include/portability/instr_time.h b/src/include/portability/instr_time.h
index 92558e234ac..4917728afc3 100644
--- a/src/include/portability/instr_time.h
+++ b/src/include/portability/instr_time.h
@@ -165,7 +165,16 @@ extern PGDLLIMPORT int32 timing_tsc_frequency_khz;
 
 extern void pg_initialize_timing_tsc(void);
 
-extern uint32 pg_tsc_calibrate_frequency(void);
+typedef struct TscClockSourceInfo
+{
+	int32		frequency_khz;	/* from CPUID or calibration */
+	int32		calibrated_frequency_khz;	/* from calibration loop, 0 if did
+											 * not converge, -1 if not yet run */
+	char		frequency_source[128];	/* describes how frequency was
+										 * determined */
+} TscClockSourceInfo;
+
+extern const TscClockSourceInfo *pg_timing_tsc_clock_source_info(void);
 
 #endif							/* PG_INSTR_TSC_CLOCK */
 
diff --git a/src/common/instr_time.c b/src/common/instr_time.c
index fc6e1852c30..2523d6a0df6 100644
--- a/src/common/instr_time.c
+++ b/src/common/instr_time.c
@@ -70,6 +70,8 @@ static void set_ticks_per_ns(void);
 static void set_ticks_per_ns_system(void);
 
 #if PG_INSTR_TSC_CLOCK
+static TscClockSourceInfo tsc_info = {.calibrated_frequency_khz = -1};
+
 static bool tsc_use_by_default(void);
 static void set_ticks_per_ns_for_tsc(void);
 #endif
@@ -166,6 +168,7 @@ set_ticks_per_ns_system(void)
 #if PG_INSTR_TSC_CLOCK
 
 static void tsc_detect_frequency(void);
+static uint32 pg_tsc_calibrate_frequency(void);
 
 /*
  * Initialize the TSC clock source by determining its usability and frequency.
@@ -202,21 +205,36 @@ static void
 tsc_detect_frequency(void)
 {
 	timing_tsc_frequency_khz = 0;
+	tsc_info.frequency_khz = 0;
+	tsc_info.frequency_source[0] = '\0';
 
 	/* We require RDTSCP support and an invariant TSC, bail if not available */
 	if (!x86_feature_available(PG_RDTSCP) || !x86_feature_available(PG_TSC_INVARIANT))
 		return;
 
 	/* Determine speed at which the TSC advances */
-	timing_tsc_frequency_khz = x86_tsc_frequency_khz();
+	timing_tsc_frequency_khz = x86_tsc_frequency_khz(tsc_info.frequency_source,
+													 sizeof(tsc_info.frequency_source));
 	if (timing_tsc_frequency_khz > 0)
+	{
+		tsc_info.frequency_khz = timing_tsc_frequency_khz;
 		return;
+	}
 
 	/*
 	 * CPUID did not give us the TSC frequency. We can instead measure the
 	 * frequency by comparing ticks against walltime in a calibration loop.
 	 */
-	timing_tsc_frequency_khz = pg_tsc_calibrate_frequency();
+	if (tsc_info.calibrated_frequency_khz < 0)
+		tsc_info.calibrated_frequency_khz = pg_tsc_calibrate_frequency();
+
+	timing_tsc_frequency_khz = tsc_info.calibrated_frequency_khz;
+	if (timing_tsc_frequency_khz > 0)
+	{
+		strlcpy(tsc_info.frequency_source, "x86, calibration",
+				sizeof(tsc_info.frequency_source));
+		tsc_info.frequency_khz = timing_tsc_frequency_khz;
+	}
 }
 
 /*
@@ -282,7 +300,7 @@ tsc_use_by_default(void)
 #define TSC_CALIBRATION_ITERATIONS	1000000
 #define TSC_CALIBRATION_SKIPS		100
 #define TSC_CALIBRATION_STABLE_CYCLES	10
-uint32
+static uint32
 pg_tsc_calibrate_frequency(void)
 {
 	instr_time	initial_wall;
@@ -369,4 +387,24 @@ pg_tsc_calibrate_frequency(void)
 	return (uint32) freq_khz;
 }
 
+/*
+ * Returns TSC clock source information for diagnostic purposes.
+ *
+ * On first call, runs the TSC calibration loop (if not already done during
+ * frequency detection) which may take up to TSC_CALIBRATION_MAX_NS.
+ * Subsequent calls return cached results.
+ *
+ * Note: This won't return the right info in EXEC_BACKEND builds if this were
+ * used in the backend (which it currently is not), as tsc_info is not copied
+ * using read_backend_variables - only the TSC frequency is.
+ */
+const TscClockSourceInfo *
+pg_timing_tsc_clock_source_info(void)
+{
+	if (tsc_info.calibrated_frequency_khz < 0)
+		tsc_info.calibrated_frequency_khz = pg_tsc_calibrate_frequency();
+
+	return &tsc_info;
+}
+
 #endif							/* PG_INSTR_TSC_CLOCK */
diff --git a/src/port/pg_cpu_x86.c b/src/port/pg_cpu_x86.c
index 150b4a1d574..c097807fc7f 100644
--- a/src/port/pg_cpu_x86.c
+++ b/src/port/pg_cpu_x86.c
@@ -13,7 +13,11 @@
  *-------------------------------------------------------------------------
  */
 
-#include "c.h"
+#ifndef FRONTEND
+#include "postgres.h"
+#else
+#include "postgres_fe.h"
+#endif
 
 #if defined(USE_SSE2) || defined(__i386__)
 
@@ -161,10 +165,13 @@ static uint32 x86_hypervisor_tsc_frequency_khz(void);
  * 0 indicates the frequency information was not accessible via CPUID.
  */
 uint32
-x86_tsc_frequency_khz(void)
+x86_tsc_frequency_khz(char *source, size_t source_len)
 {
 	unsigned int reg[4] = {0};
 
+	if (source)
+		strlcpy(source, "x86", source_len);
+
 	/*
 	 * If we're inside a virtual machine, try to fetch the TSC frequency from
 	 * the hypervisor, using a hypervisor specific method.
@@ -174,7 +181,11 @@ x86_tsc_frequency_khz(void)
 	 * to be wildly incorrect when virtualized.
 	 */
 	if (x86_feature_available(PG_HYPERVISOR))
+	{
+		if (source)
+			strlcat(source, ", hypervisor, cpuid 0x40000010", source_len);
 		return x86_hypervisor_tsc_frequency_khz();
+	}
 
 	/*
 	 * On modern Intel CPUs, the TSC is implemented by invariant timekeeping
@@ -207,6 +218,9 @@ x86_tsc_frequency_khz(void)
 		if (reg[EAX] == 0 || reg[EBX] == 0)
 			return 0;
 
+		if (source)
+			strlcat(source, ", cpuid 0x15", source_len);
+
 		return reg[ECX] / 1000 * reg[EBX] / reg[EAX];
 	}
 
@@ -217,7 +231,12 @@ x86_tsc_frequency_khz(void)
 	 */
 	pg_cpuid(0x16, reg);
 	if (reg[EAX] > 0)
+	{
+		if (source)
+			strlcat(source, ", cpuid 0x16", source_len);
+
 		return reg[EAX] * 1000;
+	}
 
 	return 0;
 }
diff --git a/src/bin/pg_test_timing/pg_test_timing.c b/src/bin/pg_test_timing/pg_test_timing.c
index 2afb0e6a410..ef5dd90c41f 100644
--- a/src/bin/pg_test_timing/pg_test_timing.c
+++ b/src/bin/pg_test_timing/pg_test_timing.c
@@ -184,7 +184,7 @@ static void
 test_tsc_timing(void)
 {
 	uint64		loop_count;
-	uint32		calibrated_freq;
+	const TscClockSourceInfo *info;
 
 	printf("\n");
 	loop_count = test_timing(test_duration, TIMING_CLOCK_SOURCE_TSC, false);
@@ -198,11 +198,13 @@ test_tsc_timing(void)
 		output(loop_count);
 		printf("\n");
 
-		printf(_("TSC frequency in use: %u kHz\n"), timing_tsc_frequency_khz);
+		info = pg_timing_tsc_clock_source_info();
+		printf(_("TSC frequency in use: %d kHz\n"), info->frequency_khz);
+		if (info->frequency_source[0] != '\0')
+			printf(_("TSC frequency source: %s\n"), info->frequency_source);
 
-		calibrated_freq = pg_tsc_calibrate_frequency();
-		if (calibrated_freq > 0)
-			printf(_("TSC frequency from calibration: %u kHz\n"), calibrated_freq);
+		if (info->calibrated_frequency_khz > 0)
+			printf(_("TSC frequency from calibration: %d kHz\n"), info->calibrated_frequency_khz);
 		else
 			printf(_("TSC calibration did not converge\n"));
 
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index ea95e7984bc..2137f1e0cfe 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -3090,6 +3090,7 @@ TParserStateActionItem
 TQueueDestReceiver
 TRGM
 TSAnyCacheEntry
+TscClockSourceInfo
 TSConfigCacheEntry
 TSConfigInfo
 TSDictInfo
-- 
2.53.0.1.gb2826b52eb

--5ynaw2hkmtmijn2o
Content-Type: text/x-diff; charset=us-ascii
Content-Disposition: attachment;
	filename="v26a-0002-fixup-instrumentation-Show-additional-TSC-clock.patch"