Date: Tue, 27 Nov 2007 15:01:08 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Jeff Dike <jdike@addtoit.com>
Cc: linux-kernel@vger.kernel.org, user-mode-linux-devel@lists.sourceforge.net,
       miklos@szeredi.hu
Subject: Re: [PATCH] UML - Fix !NO_HZ busy-loop
Message-Id: <20071127150108.fb94aaca.akpm@linux-foundation.org>
In-Reply-To: <20071127200347.GA8847@c2.user-mode-linux.org>
References: <20071127200347.GA8847@c2.user-mode-linux.org>
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On Tue, 27 Nov 2007 15:03:47 -0500
Jeff Dike <jdike@addtoit.com> wrote:

> [ This one needs to get into 2.6.24 ]
> 
> With NO_HZ disabled, the UML idle loop effectively becomes a busy
> loop, as it will sleep for no time.
> 
> The cause was forgetting to restart the tick after waking up from
> sleep.  It was disabled before sleeping, and the remaining time used
> as the interval to sleep.  So, the tick needs to be restarted when
> nanosleep finishes.
> 
> This is done by introducing after_sleep_interval, which is empty in
> the NO_HZ case, but which sets the tick starting in the !NO_HZ case.
> 
> Signed-off-by: Jeff Dike <jdike@linux.intel.com>
> ---
>  arch/um/os-Linux/time.c |   54 +++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 51 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6.22/arch/um/os-Linux/time.c
> ===================================================================
> --- linux-2.6.22.orig/arch/um/os-Linux/time.c	2007-11-14 10:33:29.000000000 -0500
> +++ linux-2.6.22/arch/um/os-Linux/time.c	2007-11-26 15:50:46.000000000 -0500
> @@ -59,7 +59,7 @@ long long disable_timer(void)
>  {
>  	struct itimerval time = ((struct itimerval) { { 0, 0 }, { 0, 0 } });
>  
> -	if(setitimer(ITIMER_VIRTUAL, &time, &time) < 0)
> +	if (setitimer(ITIMER_VIRTUAL, &time, &time) < 0)
>  		printk(UM_KERN_ERR "disable_timer - setitimer failed, "
>  		       "errno = %d\n", errno);
>  
> @@ -74,13 +74,61 @@ long long os_nsecs(void)
>  	return timeval_to_ns(&tv);
>  }
>  
> +#ifdef UML_CONFIG_NO_HZ

Nothing ever defines this?

> +static int after_sleep_interval(struct timespec *ts)
> +{
> +}
> +#else
> +static inline long long timespec_to_us(const struct timespec *ts)
> +{
> +	return ((long long) ts->tv_sec * UM_USEC_PER_SEC) +
> +		ts->tv_nsec / UM_NSEC_PER_USEC;
> +}
> +
> +static int after_sleep_interval(struct timespec *ts)
> +{
> +	int usec = UM_USEC_PER_SEC / UM_HZ;
> +	long long start_usecs = timespec_to_us(ts);
> +	struct timeval tv;
> +	struct itimerval interval;
> +
> +	/*
> +	 * It seems that rounding can increase the value returned from
> +	 * setitimer to larger than the one passed in.  Over time,
> +	 * this will cause the remaining time to be greater than the
> +	 * tick interval.  If this happens, then just reduce the first
> +	 * tick to the interval value.
> +	 */
> +	if (start_usecs > usec)
> +		start_usecs = usec;
> +	tv = ((struct timeval) { .tv_sec  = start_usecs / UM_USEC_PER_SEC,
> +				 .tv_usec = start_usecs % UM_USEC_PER_SEC });
> +	interval = ((struct itimerval) { { 0, usec }, tv });
> +
> +	if (setitimer(ITIMER_VIRTUAL, &interval, NULL) == -1)
> +		return -errno;
> +
> +	return 0;
> +}
> +#endif
> +
>  extern void alarm_handler(int sig, struct sigcontext *sc);
>  
>  void idle_sleep(unsigned long long nsecs)
>  {
> -	struct timespec ts = { .tv_sec	= nsecs / UM_NSEC_PER_SEC,
> -			       .tv_nsec = nsecs % UM_NSEC_PER_SEC };
> +	struct timespec ts;
> +
> +	/*
> +	 * nsecs can come in as zero, in which case, this starts a
> +	 * busy loop.  To prevent this, reset nsecs to the tick
> +	 * interval if it is zero.
> +	 */
> +	if (nsecs == 0)
> +		nsecs = UM_NSEC_PER_SEC / UM_HZ;
> +	ts = ((struct timespec) { .tv_sec	= nsecs / UM_NSEC_PER_SEC,
> +				  .tv_nsec	= nsecs % UM_NSEC_PER_SEC });
>  
>  	if (nanosleep(&ts, &ts) == 0)
>  		alarm_handler(SIGVTALRM, NULL);
> +	after_sleep_interval(&ts);
>  }
-
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