Subject: Re: high resolution timers, scheduling & sleep granularity
From: Peter Zijlstra <peterz@infradead.org>
To: Josef Bacik <jbacik@redhat.com>
Cc: Ric Wheeler <rwheeler@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
       Ingo Molnar <mingo@elte.hu>, linux-fsdevel@vger.kernel.org,
       Chris Mason <chris.mason@oracle.com>, linux-kernel@vger.kernel.org
In-Reply-To: <20080801132537.GB14001@unused.rdu.redhat.com>
References: <4892FC11.4020105@redhat.com>
	 <20080801132537.GB14001@unused.rdu.redhat.com>
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Date: Fri, 01 Aug 2008 16:50:11 +0200
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On Fri, 2008-08-01 at 09:25 -0400, Josef Bacik wrote:

> Index: linux-2.6/fs/jbd/transaction.c
> ===================================================================
> --- linux-2.6.orig/fs/jbd/transaction.c
> +++ linux-2.6/fs/jbd/transaction.c

> @@ -63,6 +65,32 @@ get_transaction(journal_t *journal, tran
>  	return transaction;
>  }
>  
> +static void precision_sleep(unsigned long time)
> +{
> +	struct hrtimer_sleeper t;
> +
> +	hrtimer_init_on_stack(&t.timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
> +	hrtimer_init_sleeper(&t, current);
> +	t.timer.expires = ktime_add_ns(ktime_get_real(), time);
> +
> +	do {
> +		set_current_state(TASK_UNINTERRUPTIBLE);
> +
> +		hrtimer_start(&t.timer, t.timer.expires, HRTIMER_MODE_ABS);
> +		if (!hrtimer_active(&t.timer))
> +			t.task = NULL;
> +
> +		if (likely(t.task))
> +			schedule();
> +
> +		hrtimer_cancel(&t.timer);
> +	} while (t.task);
> +
> +	set_current_state(TASK_RUNNING);
> +
> +	destroy_hrtimer_on_stack(&t.timer);
> +}
> +
>  /*
>   * Handle management.
>   *

I was convinced we already had such a creature,. but I guess I was wrong
as I can't find it ;-)

Anyway, I'm thinking this function ought to live in kernel/hrtimer.c and
possibly get renamed to something like hrtimer_sleep_ns() or some such
(means you can also reuse the do_nanosleep helper in there).

Also, have you considered the impact on platforms that do not support
hrtimers, or don't have high resolution clock events available?

> Index: linux-2.6/kernel/hrtimer.c
> ===================================================================
> --- linux-2.6.orig/kernel/hrtimer.c
> +++ linux-2.6/kernel/hrtimer.c
> @@ -1458,6 +1458,7 @@ void hrtimer_init_sleeper(struct hrtimer
>  	sl->timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
>  #endif
>  }
> +EXPORT_SYMBOL_GPL(hrtimer_init_sleeper);
>  
>  static int __sched do_nanosleep(struct hrtimer_sleeper *t, enum hrtimer_mode mode)
>  {

That also gets rid of this export..

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