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Subject[RFC PATCH] hrtimers: system-wide and per-task hrtimer slacks
Date
This patch proposes a system-wide sysctl-aware default for the
high-resolution timer slack value, which may be changed from 0
to HRTIMER_MAX_SLACK nanoseconds. Default system-wide and per-task
values are HRTIMER_DEFAULT_SLACK. Per-task value isn't inherited
across fork(); instead, newborn task uses system-wide value by
default, and newborn thread uses it's group leader value.

Signed-off-by: Dmitry Antipov <dmitry.antipov@linaro.org>
---
Documentation/sysctl/kernel.txt | 8 ++++++++
include/linux/hrtimer.h | 11 +++++++++++
include/linux/init_task.h | 2 +-
include/linux/sched.h | 11 ++++++++---
kernel/fork.c | 9 +++++++--
kernel/futex.c | 4 ++--
kernel/hrtimer.c | 10 +++++++---
kernel/sys.c | 9 ++++-----
kernel/sysctl.c | 10 ++++++++++
9 files changed, 58 insertions(+), 16 deletions(-)
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 6d78841..83b63ed 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -606,6 +606,14 @@ can be ORed together:

==============================================================

+timer_slack:
+
+This value can be used to query and set the default slack for
+high-resolution timers, in nanoseconds. The default value is 50
+microseconds, and can be changed from 0 nanoseconds to 1 millisecond.
+
+==============================================================
+
unknown_nmi_panic:

The value in this file affects behavior of handling NMI. When the
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index fd0dc30..77169b7 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -24,6 +24,16 @@
#include <linux/timer.h>
#include <linux/timerqueue.h>

+/*
+ * Default system-wide and per-task hrtimer slack, in nanoseconds.
+ */
+#define HRTIMER_DEFAULT_SLACK 50000
+
+/*
+ * Reasonable limit for hrtimer slack.
+ */
+#define HRTIMER_MAX_SLACK 1000000
+
struct hrtimer_clock_base;
struct hrtimer_cpu_base;

@@ -323,6 +333,7 @@ extern ktime_t ktime_get_monotonic_offset(void);

DECLARE_PER_CPU(struct tick_device, tick_cpu_device);

+extern int default_timer_slack_ns;

/* Exported timer functions: */

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 9c66b1a..b29be0d 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -178,7 +178,7 @@ extern struct cred init_cred;
.journal_info = NULL, \
.cpu_timers = INIT_CPU_TIMERS(tsk.cpu_timers), \
.pi_lock = __RAW_SPIN_LOCK_UNLOCKED(tsk.pi_lock), \
- .timer_slack_ns = 50000, /* 50 usec default slack */ \
+ .timer_slack_ns = HRTIMER_DEFAULT_SLACK, \
.pids = { \
[PIDTYPE_PID] = INIT_PID_LINK(PIDTYPE_PID), \
[PIDTYPE_PGID] = INIT_PID_LINK(PIDTYPE_PGID), \
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9b13f79..811e034 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1551,11 +1551,11 @@ struct task_struct {
struct latency_record latency_record[LT_SAVECOUNT];
#endif
/*
- * time slack values; these are used to round up poll() and
- * select() etc timeout values. These are in nanoseconds.
+ * High-resolution timer slack value, in nanoseconds.
+ * Used to round up poll()/select(), nanosleep, futex
+ * waiting, etc. timeout values of non-realtime tasks.
*/
unsigned long timer_slack_ns;
- unsigned long default_timer_slack_ns;

struct list_head *scm_work_list;
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
@@ -2631,6 +2631,11 @@ static inline int spin_needbreak(spinlock_t *lock)
#endif
}

+static inline unsigned long task_timer_slack(struct task_struct *tsk)
+{
+ return rt_task(tsk) ? 0 : tsk->timer_slack_ns;
+}
+
/*
* Thread group CPU time accounting.
*/
diff --git a/kernel/fork.c b/kernel/fork.c
index b77fd55..6aaff93 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1164,8 +1164,13 @@ static struct task_struct *copy_process(unsigned long clone_flags,
#if defined(SPLIT_RSS_COUNTING)
memset(&p->rss_stat, 0, sizeof(p->rss_stat));
#endif
-
- p->default_timer_slack_ns = current->timer_slack_ns;
+ /*
+ * New thread inherits the slack from the group
+ * leader. New process uses system-default slack.
+ */
+ p->timer_slack_ns = (clone_flags & CLONE_THREAD) ?
+ current->group_leader->timer_slack_ns :
+ default_timer_slack_ns;

task_io_accounting_init(&p->ioac);
acct_clear_integrals(p);
diff --git a/kernel/futex.c b/kernel/futex.c
index 1614be2..a0d302d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1887,7 +1887,7 @@ static int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
HRTIMER_MODE_ABS);
hrtimer_init_sleeper(to, current);
hrtimer_set_expires_range_ns(&to->timer, *abs_time,
- current->timer_slack_ns);
+ task_timer_slack(current));
}

retry:
@@ -2281,7 +2281,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
HRTIMER_MODE_ABS);
hrtimer_init_sleeper(to, current);
hrtimer_set_expires_range_ns(&to->timer, *abs_time,
- current->timer_slack_ns);
+ task_timer_slack(current));
}

/*
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index ae34bf5..0c56fec 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -51,6 +51,12 @@
#include <trace/events/timer.h>

/*
+ * Default hrtimer slack value, in nanoseconds. May be changed in
+ * [0..HRTIMER_MAX_SLACK] range through kernel.timer_slack sysctl.
+ */
+__read_mostly int default_timer_slack_ns = HRTIMER_DEFAULT_SLACK;
+
+/*
* The timer bases:
*
* There are more clockids then hrtimer bases. Thus, we index
@@ -1564,9 +1570,7 @@ long hrtimer_nanosleep(struct timespec *rqtp, struct timespec __user *rmtp,
int ret = 0;
unsigned long slack;

- slack = current->timer_slack_ns;
- if (rt_task(current))
- slack = 0;
+ slack = task_timer_slack(current);

hrtimer_init_on_stack(&t.timer, clockid, mode);
hrtimer_set_expires_range_ns(&t.timer, timespec_to_ktime(*rqtp), slack);
diff --git a/kernel/sys.c b/kernel/sys.c
index 4070153..ac32846 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -22,6 +22,7 @@
#include <linux/device.h>
#include <linux/key.h>
#include <linux/times.h>
+#include <linux/hrtimer.h>
#include <linux/posix-timers.h>
#include <linux/security.h>
#include <linux/dcookies.h>
@@ -1917,12 +1918,10 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
error = current->timer_slack_ns;
break;
case PR_SET_TIMERSLACK:
- if (arg2 <= 0)
- current->timer_slack_ns =
- current->default_timer_slack_ns;
- else
+ if (arg2 <= HRTIMER_MAX_SLACK)
current->timer_slack_ns = arg2;
- error = 0;
+ else
+ error = -EINVAL;
break;
case PR_MCE_KILL:
if (arg4 | arg5)
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index f487f25..2cd42c6 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -136,6 +136,7 @@ static int min_percpu_pagelist_fract = 8;

static int ngroups_max = NGROUPS_MAX;
static const int cap_last_cap = CAP_LAST_CAP;
+static const int slack_max = HRTIMER_MAX_SLACK;

#ifdef CONFIG_INOTIFY_USER
#include <linux/inotify.h>
@@ -1004,6 +1005,15 @@ static struct ctl_table kern_table[] = {
.proc_handler = proc_dointvec,
},
#endif
+ {
+ .procname = "timer_slack",
+ .data = &default_timer_slack_ns,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &slack_max,
+ },
{ }
};

--
1.7.7.6


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