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

[Darren Hart <dvhltc@us.ibm.com>]

On Wednesday 12 April 2006 02:42, Sébastien Dugué wrote:
>   Hi,
>
>   This patch modifies futex_wait() to use an hrtimer + schedule() in place
> of schedule_timeout() in an RT kernel.

I haven't seen any public discussion on this, so I just wanted to put in a 
nod.  It will greatly improve the latency for realtime applications using 
pthread_cond_timedwait().

Acked-by: Darren Hart <dvhltc@us.ibm.com>


>
>   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;
> -
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-- 
Darren Hart
IBM Linux Technology Center
Realtime Linux Team
Phone: 503 578 3185
  T/L: 775 3185
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