[RFC 1/2] cpuidle: Extract IPI based and timer based wakeup latency from idle states

Pratik Rajesh Sampat
2021-03-15T11:49:47Z

Introduce a mechanism to fire directed IPIs from a specified source CPU
to a specified target CPU and measure the difference in time incurred on
wakeup.

Also, introduce a mechanism to queue a HR timer on a specified CPU and
subsequently measure the time taken to wakeup the CPU.

Finally define a simple debugfs interface to control the knobs to fire
the IPI and Timer events on specified CPU and view their incurred idle
wakeup latencies.

Signed-off-by: Pratik Rajesh Sampat <psampat@linux.ibm.com>
---
drivers/cpuidle/Makefile | 1 +
drivers/cpuidle/test-cpuidle_latency.c | 157 +++++++++++++++++++++++++
lib/Kconfig.debug | 10 ++
3 files changed, 168 insertions(+)
create mode 100644 drivers/cpuidle/test-cpuidle_latency.c

diff --git a/drivers/cpuidle/Makefile b/drivers/cpuidle/Makefile
index 26bbc5e74123..3b4ee06a9164 100644
--- a/drivers/cpuidle/Makefile
+++ b/drivers/cpuidle/Makefile
@@ -8,6 +8,7 @@ obj-$(CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED) += coupled.o
obj-$(CONFIG_DT_IDLE_STATES) += dt_idle_states.o
obj-$(CONFIG_ARCH_HAS_CPU_RELAX) += poll_state.o
obj-$(CONFIG_HALTPOLL_CPUIDLE) += cpuidle-haltpoll.o
+obj-$(CONFIG_IDLE_LATENCY_SELFTEST) += test-cpuidle_latency.o

##################################################################################
# ARM SoC drivers
diff --git a/drivers/cpuidle/test-cpuidle_latency.c b/drivers/cpuidle/test-cpuidle_latency.c
new file mode 100644
index 000000000000..f138011ac225
--- /dev/null
+++ b/drivers/cpuidle/test-cpuidle_latency.c
@@ -0,0 +1,157 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Module-based API test facility for cpuidle latency using IPIs and timers
+ */
+
+#include <linux/debugfs.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+
+/*
+ * IPI based wakeup latencies
+ * Measure time taken for a CPU to wakeup on a IPI sent from another CPU
+ * The latency measured also includes the latency of sending the IPI
+ */
+struct latency {
+ unsigned int src_cpu;
+ unsigned int dest_cpu;
+ ktime_t time_start;
+ ktime_t time_end;
+ u64 latency_ns;
+} ipi_wakeup;
+
+static void measure_latency(void *info)
+{
+ struct latency *v;
+ ktime_t time_diff;
+
+ v = (struct latency *)info;
+ v->time_end = ktime_get();
+ time_diff = ktime_sub(v->time_end, v->time_start);
+ v->latency_ns = ktime_to_ns(time_diff);
+}
+
+void run_smp_call_function_test(unsigned int cpu)
+{
+ ipi_wakeup.src_cpu = smp_processor_id();
+ ipi_wakeup.dest_cpu = cpu;
+ ipi_wakeup.time_start = ktime_get();
+ smp_call_function_single(cpu, measure_latency, &ipi_wakeup, 1);
+}
+
+/*
+ * Timer based wakeup latencies
+ * Measure time taken for a CPU to wakeup on a timer being armed and fired
+ */
+struct timer_data {
+ unsigned int src_cpu;
+ u64 timeout;
+ ktime_t time_start;
+ ktime_t time_end;
+ struct hrtimer timer;
+ u64 timeout_diff_ns;
+} timer_wakeup;
+
+static enum hrtimer_restart timer_called(struct hrtimer *hrtimer)
+{
+ struct timer_data *w;
+ ktime_t time_diff;
+
+ w = container_of(hrtimer, struct timer_data, timer);
+ w->time_end = ktime_get();
+
+ time_diff = ktime_sub(w->time_end, w->time_start);
+ time_diff = ktime_sub(time_diff, ns_to_ktime(w->timeout));
+ w->timeout_diff_ns = ktime_to_ns(time_diff);
+ return HRTIMER_NORESTART;
+}
+
+static void run_timer_test(unsigned int ns)
+{
+ hrtimer_init(&timer_wakeup.timer, CLOCK_MONOTONIC,
+ HRTIMER_MODE_REL);
+ timer_wakeup.timer.function = timer_called;
+ timer_wakeup.src_cpu = smp_processor_id();
+ timer_wakeup.timeout = ns;
+ timer_wakeup.time_start = ktime_get();
+
+ hrtimer_start(&timer_wakeup.timer, ns_to_ktime(ns),
+ HRTIMER_MODE_REL_PINNED);
+}
+
+static struct dentry *dir;
+
+static int cpu_read_op(void *data, u64 *dest_cpu)
+{
+ *dest_cpu = ipi_wakeup.dest_cpu;
+ return 0;
+}
+
+static int cpu_write_op(void *data, u64 value)
+{
+ run_smp_call_function_test(value);
+ return 0;
+}
+DEFINE_SIMPLE_ATTRIBUTE(ipi_ops, cpu_read_op, cpu_write_op, "%llu\n");
+
+static int timeout_read_op(void *data, u64 *timeout)
+{
+ *timeout = timer_wakeup.timeout;
+ return 0;
+}
+
+static int timeout_write_op(void *data, u64 value)
+{
+ run_timer_test(value);
+ return 0;
+}
+DEFINE_SIMPLE_ATTRIBUTE(timeout_ops, timeout_read_op, timeout_write_op, "%llu\n");
+
+static int __init latency_init(void)
+{
+ struct dentry *temp;
+
+ dir = debugfs_create_dir("latency_test", 0);
+ if (!dir) {
+ pr_alert("latency_test: failed to create /sys/kernel/debug/latency_test\n");
+ return -1;
+ }
+ temp = debugfs_create_file("ipi_cpu_dest",
+ 0666,
+ dir,
+ NULL,
+ &ipi_ops);
+ if (!temp) {
+ pr_alert("latency_test: failed to create /sys/kernel/debug/ipi_cpu_dest\n");
+ return -1;
+ }
+ debugfs_create_u64("ipi_latency_ns", 0444, dir, &ipi_wakeup.latency_ns);
+ debugfs_create_u32("ipi_cpu_src", 0444, dir, &ipi_wakeup.src_cpu);
+
+ temp = debugfs_create_file("timeout_expected_ns",
+ 0666,
+ dir,
+ NULL,
+ &timeout_ops);
+ if (!temp) {
+ pr_alert("latency_test: failed to create /sys/kernel/debug/timeout_expected_ns\n");
+ return -1;
+ }
+ debugfs_create_u64("timeout_diff_ns", 0444, dir, &timer_wakeup.timeout_diff_ns);
+ debugfs_create_u32("timeout_cpu_src", 0444, dir, &timer_wakeup.src_cpu);
+ pr_info("Latency Test module loaded\n");
+ return 0;
+}
+
+static void __exit latency_cleanup(void)
+{
+ pr_info("Cleaning up Latency Test module.\n");
+ debugfs_remove_recursive(dir);
+}
+
+module_init(latency_init);
+module_exit(latency_cleanup);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("IBM Corporation");
+MODULE_DESCRIPTION("Measuring idle latency for IPIs and Timers");
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 7937265ef879..3d6baea16913 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1467,6 +1467,16 @@ config DEBUG_KOBJECT
If you say Y here, some extra kobject debugging messages will be sent
to the syslog.

+config IDLE_LATENCY_SELFTEST
+ tristate "Cpuidle latency selftests"
+ depends on CPU_IDLE
+ help
+ This option provides a kernel module that runs tests using the IPI and
+ timers to measure latency.
+
+ Say M if you want these self tests to build as a module.
+ Say N if you are unsure.
+
config DEBUG_KOBJECT_RELEASE
bool "kobject release debugging"
depends on DEBUG_OBJECTS_TIMERS
--
2.17.1