linux-kernel - Re: [PATCH 12/13] hrtimer: create a "timer

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Date:	Tue, 2 Sep 2008 12:04:39 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	dwmw2@...radead.org, drepper@...hat.com, mingo@...e.hu,
	tglx@...x.de
Subject: Re: [PATCH 12/13] hrtimer: create a "timer_slack" field in the
	task struct

Hi!

> From: Arjan van de Ven <arjan@...ux.intel.com>
> Subject: [PATCH] hrtimer: create a "timer_slack" field in the task struct
> 
> We want to be able to control the default "rounding" that is used by
> select() and poll() and friends. This is a per process property
> (so that we can have a "nice" like program to start certain programs with
> a looser or stricter rounding) that can be set/get via a prctl().
> 
> For this purpose, a field called "timer_slack_ns" is added to the task
> struct. In addition, a field called "default_timer_slack"ns" is added
> so that tasks easily can temporarily to a more/less accurate slack and then
> back to the default.

Is this a good idea? IMO it should be per-syscall, not per
application. Threads would certainly like private values... and this
makes really ugly interface.

...plus it bloats task struct.

...where did the sys_indirect proposals go? We created new syscalls,
right?

IMO we should create new syscalls here, too.

								Pavel

> Signed-off-by: Arjan van de Ven <arjan@...ux.intel.com>
> ---
>  include/linux/init_task.h |    1 +
>  include/linux/prctl.h     |    7 +++++++
>  include/linux/sched.h     |    6 ++++++
>  kernel/fork.c             |    2 ++
>  kernel/sys.c              |   10 ++++++++++
>  5 files changed, 26 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> index 021d8e7..23fd890 100644
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -170,6 +170,7 @@ extern struct group_info init_groups;
>  	.cpu_timers	= INIT_CPU_TIMERS(tsk.cpu_timers),		\
>  	.fs_excl	= ATOMIC_INIT(0),				\
>  	.pi_lock	= __SPIN_LOCK_UNLOCKED(tsk.pi_lock),		\
> +	.timer_slack_ns = 50000, /* 50 usec default slack */		\
>  	.pids = {							\
>  		[PIDTYPE_PID]  = INIT_PID_LINK(PIDTYPE_PID),		\
>  		[PIDTYPE_PGID] = INIT_PID_LINK(PIDTYPE_PGID),		\
> diff --git a/include/linux/prctl.h b/include/linux/prctl.h
> index 5ad7919..48d887e 100644
> --- a/include/linux/prctl.h
> +++ b/include/linux/prctl.h
> @@ -78,4 +78,11 @@
>  #define PR_GET_SECUREBITS 27
>  #define PR_SET_SECUREBITS 28
>  
> +/*
> + * Get/set the timerslack as used by poll/select/nanosleep
> + * A value of 0 means "use default"
> + */
> +#define PR_SET_TIMERSLACK 29
> +#define PR_GET_TIMERSLACK 30
> +
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index cfb0d87..f357780 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1301,6 +1301,12 @@ struct task_struct {
>  	int latency_record_count;
>  	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.
> +	 */
> +	unsigned long timer_slack_ns;
> +	unsigned long default_timer_slack_ns;
>  };
>  
>  /*
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 7ce2ebe..4308d75 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -987,6 +987,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>  	p->prev_utime = cputime_zero;
>  	p->prev_stime = cputime_zero;
>  
> +	p->default_timer_slack_ns = current->timer_slack_ns;
> +
>  #ifdef CONFIG_DETECT_SOFTLOCKUP
>  	p->last_switch_count = 0;
>  	p->last_switch_timestamp = 0;
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 038a7bc..1b96401 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1727,6 +1727,16 @@ asmlinkage long sys_prctl(int option, unsigned long arg2, unsigned long arg3,
>  		case PR_SET_TSC:
>  			error = SET_TSC_CTL(arg2);
>  			break;
> +		case PR_GET_TIMERSLACK:
> +			error = current->timer_slack_ns;
> +			break;
> +		case PR_SET_TIMERSLACK:
> +			if (arg2 <= 0)
> +				current->timer_slack_ns =
> +					current->default_timer_slack_ns;
> +			else
> +				current->timer_slack_ns = arg2;
> +			break;
>  		default:
>  			error = -EINVAL;
>  			break;

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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