timer: convert timer_slack_ns from unsigned long to u64

This patchset introduces a /proc/<pid>/timerslack_ns interface which
would allow controlling processes to be able to set the timerslack value
on other processes in order to save power by avoiding wakeups (Something
Android currently does via out-of-tree patches).

The first patch tries to fix the internal timer_slack_ns usage which was
defined as a long, which limits the slack range to ~4 seconds on 32bit
systems.  It converts it to a u64, which provides the same basically
unlimited slack (500 years) on both 32bit and 64bit machines.

The second patch introduces the /proc/<pid>/timerslack_ns interface
which allows the full 64bit slack range for a task to be read or set on
both 32bit and 64bit machines.

With these two patches, on a 32bit machine, after setting the slack on
bash to 10 seconds:

$ time sleep 1

real    0m10.747s
user    0m0.001s
sys     0m0.005s

The first patch is a little ugly, since I had to chase the slack delta
arguments through a number of functions converting them to u64s.  Let me
know if it makes sense to break that up more or not.

Other than that things are fairly straightforward.

This patch (of 2):

The timer_slack_ns value in the task struct is currently a unsigned
long.  This means that on 32bit applications, the maximum slack is just
over 4 seconds.  However, on 64bit machines, its much much larger (~500
years).

This disparity could make application development a little (as well as
the default_slack) to a u64.  This means both 32bit and 64bit systems
have the same effective internal slack range.

Now the existing ABI via PR_GET_TIMERSLACK and PR_SET_TIMERSLACK specify
the interface as a unsigned long, so we preserve that limitation on
32bit systems, where SET_TIMERSLACK can only set the slack to a unsigned
long value, and GET_TIMERSLACK will return ULONG_MAX if the slack is
actually larger then what can be stored by an unsigned long.

This patch also modifies hrtimer functions which specified the slack
delta as a unsigned long.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Oren Laadan <orenl@cellrox.com>
Cc: Ruchi Kandoi <kandoiruchi@google.com>
Cc: Rom Lemarchand <romlem@android.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 10 insertions, 7 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index 78947de..cf8ba54 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2169,7 +2169,10 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		error = perf_event_task_enable();
 		break;
 	case PR_GET_TIMERSLACK:
-		error = current->timer_slack_ns;
+		if (current->timer_slack_ns > ULONG_MAX)
+			error = ULONG_MAX;
+		else
+			error = current->timer_slack_ns;
 		break;
 	case PR_SET_TIMERSLACK:
 		if (arg2 <= 0)
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index fa909f9..58a321c 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -979,7 +979,7 @@ static inline ktime_t hrtimer_update_lowres(struct hrtimer *timer, ktime_t tim,
  *		relative (HRTIMER_MODE_REL)
  */
 void hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
-			    unsigned long delta_ns, const enum hrtimer_mode mode)
+			    u64 delta_ns, const enum hrtimer_mode mode)
 {
 	struct hrtimer_clock_base *base, *new_base;
 	unsigned long flags;
@@ -1548,7 +1548,7 @@ long hrtimer_nanosleep(struct timespec *rqtp, struct timespec __user *rmtp,
 	struct restart_block *restart;
 	struct hrtimer_sleeper t;
 	int ret = 0;
-	unsigned long slack;
+	u64 slack;
 
 	slack = current->timer_slack_ns;
 	if (dl_task(current) || rt_task(current))
@@ -1724,7 +1724,7 @@ void __init hrtimers_init(void)
  * @clock:	timer clock, CLOCK_MONOTONIC or CLOCK_REALTIME
  */
 int __sched
-schedule_hrtimeout_range_clock(ktime_t *expires, unsigned long delta,
+schedule_hrtimeout_range_clock(ktime_t *expires, u64 delta,
 			       const enum hrtimer_mode mode, int clock)
 {
 	struct hrtimer_sleeper t;
@@ -1792,7 +1792,7 @@ schedule_hrtimeout_range_clock(ktime_t *expires, unsigned long delta,
  *
  * Returns 0 when the timer has expired otherwise -EINTR
  */
-int __sched schedule_hrtimeout_range(ktime_t *expires, unsigned long delta,
+int __sched schedule_hrtimeout_range(ktime_t *expires, u64 delta,
 				     const enum hrtimer_mode mode)
 {
 	return schedule_hrtimeout_range_clock(expires, delta, mode,
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index bbc5d11..d1798fa 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1698,10 +1698,10 @@ EXPORT_SYMBOL(msleep_interruptible);
 static void __sched do_usleep_range(unsigned long min, unsigned long max)
 {
 	ktime_t kmin;
-	unsigned long delta;
+	u64 delta;
 
 	kmin = ktime_set(0, min * NSEC_PER_USEC);
-	delta = (max - min) * NSEC_PER_USEC;
+	delta = (u64)(max - min) * NSEC_PER_USEC;
 	schedule_hrtimeout_range(&kmin, delta, HRTIMER_MODE_REL);
 }