Re: [PATCH] softlockup: remove timestamp checking from hung_task

[Frederic Weisbecker]

Hi Mandeep,

On Fri, Feb 06, 2009 at 03:37:47PM -0800, Mandeep Singh Baines wrote:
> Patch against tip/core/softlockup
> 
> ---
> Impact: saves sizeof(long) bytes per task_struct
> 
> By guaranteeing that sysctl_hung_task_timeout_secs have elapsed between
> tasklist scans we can avoid using timestamps.
> 
> Signed-off-by: Mandeep Singh Baines <msb@xxxxxxxxxx>


Good idea.
BTW, why haven't you put your name on top of this file?
That would help those who will send patches knowing to whom they have to
route their mails.

I made some comments below about small things...

> ---
>  include/linux/sched.h |    1 -
>  kernel/fork.c         |    8 +++-----
>  kernel/hung_task.c    |   48 +++++++++---------------------------------------
>  3 files changed, 12 insertions(+), 45 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2a2811c..e0d723f 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1241,7 +1241,6 @@ struct task_struct {
>  #endif
>  #ifdef CONFIG_DETECT_HUNG_TASK
>  /* hung task detection */
> -	unsigned long last_switch_timestamp;
>  	unsigned long last_switch_count;
>  #endif
>  /* CPU-specific state of this task */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index fb94442..bf582f7 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -639,6 +639,9 @@ static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)
>  
>  	tsk->min_flt = tsk->maj_flt = 0;
>  	tsk->nvcsw = tsk->nivcsw = 0;
> +#ifdef CONFIG_DETECT_HUNG_TASK
> +	tsk->last_switch_count = tsk->nvcsw + tsk->nivcsw;
> +#endif


I think you can directly assign a zero here :-)
Or you want to let it as is to give some sense and explanation
about the role of this field?
Why not, I guess gcc will optimize it anyway.


>  	tsk->mm = NULL;
>  	tsk->active_mm = NULL;
> @@ -1041,11 +1044,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>  
>  	p->default_timer_slack_ns = current->timer_slack_ns;
>  
> -#ifdef CONFIG_DETECT_HUNG_TASK
> -	p->last_switch_count = 0;
> -	p->last_switch_timestamp = 0;
> -#endif
> -
>  	task_io_accounting_init(&p->ioac);
>  	acct_clear_integrals(p);
>  
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 3951a80..4a10756 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -34,7 +34,6 @@ unsigned long __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
>   * Zero means infinite timeout - no checking done:
>   */
>  unsigned long __read_mostly sysctl_hung_task_timeout_secs = 120;
> -static unsigned long __read_mostly hung_task_poll_jiffies;
>  
>  unsigned long __read_mostly sysctl_hung_task_warnings = 10;
>  
> @@ -69,33 +68,17 @@ static struct notifier_block panic_block = {
>  	.notifier_call = hung_task_panic,
>  };
>  
> -/*
> - * Returns seconds, approximately.  We don't need nanosecond
> - * resolution, and we don't need to waste time with a big divide when
> - * 2^30ns == 1.074s.
> - */
> -static unsigned long get_timestamp(void)
> -{
> -	int this_cpu = raw_smp_processor_id();
> -
> -	return cpu_clock(this_cpu) >> 30LL;  /* 2^30 ~= 10^9 */
> -}
> -
> -static void check_hung_task(struct task_struct *t, unsigned long now,
> -			    unsigned long timeout)
> +static void check_hung_task(struct task_struct *t, unsigned long timeout)
>  {
>  	unsigned long switch_count = t->nvcsw + t->nivcsw;
>  
>  	if (t->flags & PF_FROZEN)
>  		return;
>  
> -	if (switch_count != t->last_switch_count || !t->last_switch_timestamp) {
> +	if (switch_count != t->last_switch_count) {
>  		t->last_switch_count = switch_count;
> -		t->last_switch_timestamp = now;
>  		return;
>  	}



What happens here if khungtaskd is scheduled in after tsk is inserted on the task_list
in copy_process() but before tsk has been scheduled once?

tsk->last_switch_count and  tsk->nvcsw + tsk->nivcsw will still be equal to zero right?

Perhaps you could add another check such as

if (!switch_count)
	return;


> -	if ((long)(now - t->last_switch_timestamp) < timeout)
> -		return;
>  	if (!sysctl_hung_task_warnings)
>  		return;
>  	sysctl_hung_task_warnings--;
> @@ -111,7 +94,6 @@ static void check_hung_task(struct task_struct *t, unsigned long now,
>  	sched_show_task(t);
>  	__debug_show_held_locks(t);
>  
> -	t->last_switch_timestamp = now;
>  	touch_nmi_watchdog();
>  
>  	if (sysctl_hung_task_panic)
> @@ -145,7 +127,6 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  {
>  	int max_count = sysctl_hung_task_check_count;
>  	int batch_count = HUNG_TASK_BATCHING;
> -	unsigned long now = get_timestamp();
>  	struct task_struct *g, *t;
>  
>  	/*
> @@ -168,19 +149,16 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>  		}
>  		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
>  		if (t->state == TASK_UNINTERRUPTIBLE)
> -			check_hung_task(t, now, timeout);
> +			check_hung_task(t, timeout);
>  	} while_each_thread(g, t);
>   unlock:
>  	rcu_read_unlock();
>  }
>  
> -static void update_poll_jiffies(void)
> +static unsigned long timeout_jiffies(unsigned long timeout)
>  {
>  	/* timeout of 0 will disable the watchdog */
> -	if (sysctl_hung_task_timeout_secs == 0)
> -		hung_task_poll_jiffies = MAX_SCHEDULE_TIMEOUT;
> -	else
> -		hung_task_poll_jiffies = sysctl_hung_task_timeout_secs * HZ / 2;
> +	return (timeout ? timeout * HZ : MAX_SCHEDULE_TIMEOUT);
>  }
>  
>  /*
> @@ -197,8 +175,6 @@ int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
>  	if (ret || !write)
>  		goto out;
>  
> -	update_poll_jiffies();
> -
>  	wake_up_process(watchdog_task);


I'm not sure what does this function now that you dropped update_poll_jiffies()
So if the user sets up a new timeout value, the only effect will be that khungtaskd will
be awakened?

But actually the /sys file doesn't seem to be set up.

Other than these comments, that looks good!
Thanks.

Frederic.


>  
>   out:
> @@ -211,20 +187,14 @@ int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
>  static int watchdog(void *dummy)
>  {
>  	set_user_nice(current, 0);
> -	update_poll_jiffies();
>  
>  	for ( ; ; ) {
> -		unsigned long timeout;
> +		unsigned long timeout = sysctl_hung_task_timeout_secs;
>  
> -		while (schedule_timeout_interruptible(hung_task_poll_jiffies));
> +		while (schedule_timeout_interruptible(timeout_jiffies(timeout)))
> +			timeout = sysctl_hung_task_timeout_secs;
>  
> -		/*
> -		 * Need to cache timeout here to avoid timeout being set
> -		 * to 0 via sysctl while inside check_hung_*_tasks().
> -		 */
> -		timeout = sysctl_hung_task_timeout_secs;
> -		if (timeout)
> -			check_hung_uninterruptible_tasks(timeout);
> +		check_hung_uninterruptible_tasks(timeout);
>  	}
>  
>  	return 0;
> -- 
> 1.5.4.5
> 

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