[RFC][PATCH -rt] Make futex_wait() use an hrtimer for timeout

Sébastien Dugué
2006-04-12T09:38:58Z

Hi,

This patch modifies futex_wait() to use an hrtimer + schedule() in place of
schedule_timeout() in an RT kernel.

More details in the patch header.

Sébastien.

--------------------------------------------------------------------------------

This patch modifies futex_wait() to use an hrtimer + schedule() in place of
schedule_timeout().

schedule_timeout() is tick based, therefore the timeout granularity is
the tick (1 ms, 4 ms or 10 ms depending on HZ). By using a high resolution
timer for timeout wakeup, we can attain a much finer timeout granularity
(in the microsecond range). This parallels what is already done for
futex_lock_pi().

The timeout passed to the syscall is no longer converted to jiffies
and is therefore passed to do_futex() and futex_wait() as a timespec
therefore keeping nanosecond resolution.

Also this removes the need to pass the nanoseconds timeout part to
futex_lock_pi() in val2.

In futex_wait(), if the timeout is zero then a regular schedule() is
performed. Otherwise, an hrtimer is fired before schedule() is called.

include/linux/futex.h | 2 -
kernel/futex.c | 56 +++++++++++++++++++++++++++++++++-----------------
kernel/futex_compat.c | 9 --------
3 files changed, 40 insertions(+), 27 deletions(-)

Signed-off-by: Sébastien Dugué <sebastien.dugue@bull.net>

Index: linux-2.6.16-rt16/include/linux/futex.h
===================================================================
--- linux-2.6.16-rt16.orig/include/linux/futex.h 2006-04-12 11:20:57.000000000 +0200
+++ linux-2.6.16-rt16/include/linux/futex.h 2006-04-12 11:21:01.000000000 +0200
@@ -93,7 +93,7 @@ struct robust_list_head {
*/
#define ROBUST_LIST_LIMIT 2048

-long do_futex(u32 __user *uaddr, int op, u32 val, unsigned long timeout,
+long do_futex(u32 __user *uaddr, int op, u32 val, struct timespec *timeout,
u32 __user *uaddr2, u32 val2, u32 val3);

extern int handle_futex_death(u32 __user *uaddr, struct task_struct *curr);
Index: linux-2.6.16-rt16/kernel/futex.c
===================================================================
--- linux-2.6.16-rt16.orig/kernel/futex.c 2006-04-12 11:20:57.000000000 +0200
+++ linux-2.6.16-rt16/kernel/futex.c 2006-04-12 11:21:01.000000000 +0200
@@ -49,6 +49,7 @@
#include <linux/syscalls.h>
#include <linux/signal.h>
#include <asm/futex.h>
+#include <linux/hrtimer.h>

#include "rtmutex_common.h"

@@ -968,7 +969,7 @@ static void unqueue_me_pi(struct futex_q
drop_key_refs(&q->key);
}

-static int futex_wait(u32 __user *uaddr, u32 val, unsigned long time)
+static int futex_wait(u32 __user *uaddr, u32 val, struct timespec *time)
{
struct task_struct *curr = current;
DECLARE_WAITQUEUE(wait, curr);
@@ -976,6 +977,8 @@ static int futex_wait(u32 __user *uaddr,
struct futex_q q;
u32 uval;
int ret;
+ struct hrtimer_sleeper t;
+ int rem = 0;

q.pi_state = NULL;
retry:
@@ -1057,7 +1060,31 @@ static int futex_wait(u32 __user *uaddr,
unsigned long nosched_flag = current->flags & PF_NOSCHED;

current->flags &= ~PF_NOSCHED;
- time = schedule_timeout(time);
+
+ if (time->tv_sec == 0 && time->tv_nsec == 0)
+ schedule();
+ else {
+
+ hrtimer_init(&t.timer, CLOCK_MONOTONIC, HRTIMER_REL);
+ hrtimer_init_sleeper(&t, current);
+ t.timer.expires = timespec_to_ktime(*time);
+
+ hrtimer_start(&t.timer, t.timer.expires, HRTIMER_REL);
+
+ /*
+ * the timer could have already expired, in which
+ * case current would be flagged for rescheduling.
+ * Don't bother calling schedule.
+ */
+ if (likely(t.task))
+ schedule();
+
+ hrtimer_cancel(&t.timer);
+
+ /* Flag if a timeout occured */
+ rem = (t.task == NULL);
+ }
+
current->flags |= nosched_flag;
}
__set_current_state(TASK_RUNNING);
@@ -1070,7 +1097,7 @@ static int futex_wait(u32 __user *uaddr,
/* If we were woken (and unqueued), we succeeded, whatever. */
if (!unqueue_me(&q))
return 0;
- if (time == 0)
+ if (rem)
return -ETIMEDOUT;
/*
* We expect signal_pending(current), but another thread may
@@ -1093,7 +1120,7 @@ static int futex_wait(u32 __user *uaddr,
* races the kernel might see a 0 value of the futex too.)
*/
static int futex_lock_pi(u32 __user *uaddr, int detect,
- unsigned long sec, long nsec, int trylock)
+ struct timespec *time, int trylock)
{
struct task_struct *curr = current;
struct futex_hash_bucket *hb;
@@ -1107,11 +1134,11 @@ static int futex_lock_pi(u32 __user *uad

pr_debug("Futex lock pi: %d\n", current->pid);

- if (sec != MAX_SCHEDULE_TIMEOUT) {
+ if (time->tv_sec || time->tv_nsec) {
to = &timeout;
hrtimer_init(&to->timer, CLOCK_REALTIME, HRTIMER_ABS);
hrtimer_init_sleeper(to, current);
- to->timer.expires = ktime_set(sec, nsec);
+ to->timer.expires = timespec_to_ktime(*time);
}

q.pi_state = NULL;
@@ -1708,7 +1735,7 @@ void exit_robust_list(struct task_struct
}
}

-long do_futex(u32 __user *uaddr, int op, u32 val, unsigned long timeout,
+long do_futex(u32 __user *uaddr, int op, u32 val, struct timespec *timeout,
u32 __user *uaddr2, u32 val2, u32 val3)
{
int ret;
@@ -1734,13 +1761,13 @@ long do_futex(u32 __user *uaddr, int op,
ret = futex_wake_op(uaddr, uaddr2, val, val2, val3);
break;
case FUTEX_LOCK_PI:
- ret = futex_lock_pi(uaddr, val, timeout, val2, 0);
+ ret = futex_lock_pi(uaddr, val, timeout, 0);
break;
case FUTEX_UNLOCK_PI:
ret = futex_unlock_pi(uaddr);
break;
case FUTEX_TRYLOCK_PI:
- ret = futex_lock_pi(uaddr, 0, timeout, val2, 1);
+ ret = futex_lock_pi(uaddr, 0, timeout, 1);
break;
default:
ret = -ENOSYS;
@@ -1753,8 +1780,7 @@ asmlinkage long sys_futex(u32 __user *ua
struct timespec __user *utime, u32 __user *uaddr2,
u32 val3)
{
- struct timespec t;
- unsigned long timeout = MAX_SCHEDULE_TIMEOUT;
+ struct timespec t = {.tv_sec=0, .tv_nsec=0};
u32 val2 = 0;

if (utime && (op == FUTEX_WAIT || op == FUTEX_LOCK_PI)) {
@@ -1762,12 +1788,6 @@ asmlinkage long sys_futex(u32 __user *ua
return -EFAULT;
if (!timespec_valid(&t))
return -EINVAL;
- if (op == FUTEX_WAIT)
- timeout = timespec_to_jiffies(&t) + 1;
- else {
- timeout = t.tv_sec;
- val2 = t.tv_nsec;
- }
}
/*
* requeue parameter in 'utime' if op == FUTEX_REQUEUE.
@@ -1775,7 +1795,7 @@ asmlinkage long sys_futex(u32 __user *ua
if (op == FUTEX_REQUEUE || op == FUTEX_CMP_REQUEUE)
val2 = (u32) (unsigned long) utime;

- return do_futex(uaddr, op, val, timeout, uaddr2, val2, val3);
+ return do_futex(uaddr, op, val, &t, uaddr2, val2, val3);
}

static struct super_block *
Index: linux-2.6.16-rt16/kernel/futex_compat.c
===================================================================
--- linux-2.6.16-rt16.orig/kernel/futex_compat.c 2006-04-12 11:20:57.000000000 +0200
+++ linux-2.6.16-rt16/kernel/futex_compat.c 2006-04-12 11:21:01.000000000 +0200
@@ -125,8 +125,7 @@ asmlinkage long compat_sys_futex(u32 __u
struct compat_timespec __user *utime, u32 __user *uaddr2,
u32 val3)
{
- struct timespec t;
- unsigned long timeout = MAX_SCHEDULE_TIMEOUT;
+ struct timespec t = {.tv_sec=0, .tv_nsec=0};
int val2 = 0;

if (utime && (op == FUTEX_WAIT || op == FUTEX_LOCK_PI)) {
@@ -134,12 +133,6 @@ asmlinkage long compat_sys_futex(u32 __u
return -EFAULT;
if (!timespec_valid(&t))
return -EINVAL;
- if (op == FUTEX_WAIT)
- timeout = timespec_to_jiffies(&t) + 1;
- else {
- timeout = t.tv_sec;
- val2 = t.tv_nsec;
- }
}
if (op == FUTEX_REQUEUE || op == FUTEX_CMP_REQUEUE)
val2 = (int) (unsigned long) utime;