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Subject[PATCH 3/6] Expose do_timer CPU as RO variable to userspace via sysctl
Date
From: Henrik Austad <haustad@cisco.com>

This allows userspace to see which CPU is currently responsible for
handling the do_timer update of the time machinery.

sysctl kernel.current_timer_cpu
/proc/sys/kernel/current_timer_cpu

Note that this value can be fleeting if CONFIG_NO_HZ_FULL is enabled. If
not read, no additional overhead is generated in the system.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: John Stultz <john.stultz@linaro.org>
Signed-off-by: Henrik Austad <haustad@cisco.com>
---
include/linux/clocksource.h | 11 +++++++++++
kernel/sysctl.c | 8 ++++++++
kernel/time/timekeeping.c | 21 +++++++++++++++++++++
3 files changed, 40 insertions(+)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 67301a4..cfd39e8 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -8,6 +8,7 @@
#ifndef _LINUX_CLOCKSOURCE_H
#define _LINUX_CLOCKSOURCE_H

+#include <linux/sysctl.h>
#include <linux/types.h>
#include <linux/timex.h>
#include <linux/time.h>
@@ -358,4 +359,14 @@ static inline void clocksource_of_init(void) {}
.data = (fn == (clocksource_of_init_fn)NULL) ? fn : fn }
#endif

+/*
+ * expose the CPU that handles the timer-tick.
+ */
+extern int expose_tick_do_timer_cpu;
+extern int timekeeping_expose_timer_cpu(struct ctl_table *table,
+ int write,
+ void __user *buffer,
+ size_t *lenp,
+ loff_t *ppos);
+
#endif /* _LINUX_CLOCKSOURCE_H */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 49e13e1..a882c9e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -63,6 +63,7 @@
#include <linux/binfmts.h>
#include <linux/sched/sysctl.h>
#include <linux/kexec.h>
+#include <linux/clocksource.h>

#include <asm/uaccess.h>
#include <asm/processor.h>
@@ -424,6 +425,13 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = sched_rr_handler,
},
+ {
+ .procname = "current_timer_cpu",
+ .data = &expose_tick_do_timer_cpu,
+ .maxlen = sizeof(int),
+ .mode = 0444,
+ .proc_handler = timekeeping_expose_timer_cpu,
+ },
#ifdef CONFIG_SCHED_AUTOGROUP
{
.procname = "sched_autogroup_enabled",
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index f7c6b1f..55428f9 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -30,6 +30,13 @@
#include "timekeeping_internal.h"

/*
+ * Hold the current value of tick_do_timer_cpu. We could expose this directly,
+ * but putting this will prevent a casual update of the mode in sysctl.c to
+ * suddenly change the timer-cpu.
+ */
+int expose_tick_do_timer_cpu;
+
+/*
* sysfs interface to timer-cpu
*/
static ssize_t current_cpu_show(struct kobject *kobj,
@@ -1782,3 +1789,17 @@ void xtime_update(unsigned long ticks)
write_sequnlock(&jiffies_lock);
update_wall_time();
}
+
+/*
+ * sysctl interface for exposing timer tick CPU
+ */
+int timekeeping_expose_timer_cpu(struct ctl_table *table,
+ int write,
+ void __user *buffer,
+ size_t *lenp,
+ loff_t *ppos)
+{
+ /* proc_dointvec will update the buffer written userspace. */
+ expose_tick_do_timer_cpu = tick_expose_cpu();
+ return proc_dointvec(table, write, buffer, lenp, ppos);
+}
--
1.7.9.5


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