Re: [PATCH] sched/core: make "Preemption disabled at" message more useful

[Ingo Molnar <mingo@kernel.org>]

* Vegard Nossum <vegard.nossum@oracle.com> wrote:

> This message is currently really useless since it always prints a value
> that comes from the printk() we just did, e.g.:
> 
>     BUG: sleeping function called from invalid context at mm/slab.h:388
>     in_atomic(): 0, irqs_disabled(): 0, pid: 31996, name: trinity-c1
>     Preemption disabled at:[<ffffffff8119db33>] down_trylock+0x13/0x80
> 
>     BUG: sleeping function called from invalid context at include/linux/freezer.h:56
>     in_atomic(): 0, irqs_disabled(): 0, pid: 31996, name: trinity-c1
>     Preemption disabled at:[<ffffffff811aaa37>] console_unlock+0x2f7/0x930
> 
> Here, both down_trylock() and console_unlock() is somewhere in the
> printk() path.
> 
> We should save the value before calling printk() and use the saved value
> instead. That immediately reveals the offending callsite:
> 
>     BUG: sleeping function called from invalid context at mm/slab.h:388
>     in_atomic(): 0, irqs_disabled(): 0, pid: 14971, name: trinity-c2
>     Preemption disabled at:[<ffffffff819bcd46>] rhashtable_walk_start+0x46/0x150
> 
> (Bug report: http://marc.info/?l=linux-netdev&m=146925979821849&w=2)
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Rusty Russel <rusty@rustcorp.com.au>
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
> ---
>  kernel/sched/core.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7171cf9..87689a6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3144,9 +3144,18 @@ static inline void preempt_latency_stop(int val) { }
>   */
>  static noinline void __schedule_bug(struct task_struct *prev)
>  {
> +#ifdef CONFIG_DEBUG_PREEMPT
> +	unsigned long preempt_disable_ip;
> +#endif
> +
>  	if (oops_in_progress)
>  		return;
>  
> +#ifdef CONFIG_DEBUG_PREEMPT
> +	/* Save this before calling printk(), since that will clobber it */
> +	preempt_disable_ip = current->preempt_disable_ip;
> +#endif

These two blocks could be merged trivially, avoiding an #ifdef pair ...

> +
>  	printk(KERN_ERR "BUG: scheduling while atomic: %s/%d/0x%08x\n",
>  		prev->comm, prev->pid, preempt_count());
>  
> @@ -3157,7 +3166,7 @@ static noinline void __schedule_bug(struct task_struct *prev)
>  #ifdef CONFIG_DEBUG_PREEMPT
>  	if (in_atomic_preempt_off()) {
>  		pr_err("Preemption disabled at:");
> -		print_ip_sym(current->preempt_disable_ip);
> +		print_ip_sym(preempt_disable_ip);
>  		pr_cont("\n");
>  	}
>  #endif
> @@ -7541,6 +7550,9 @@ EXPORT_SYMBOL(__might_sleep);
>  void ___might_sleep(const char *file, int line, int preempt_offset)
>  {
>  	static unsigned long prev_jiffy;	/* ratelimiting */
> +#ifdef CONFIG_DEBUG_PREEMPT
> +	unsigned long preempt_disable_ip;
> +#endif
>  
>  	rcu_sleep_check(); /* WARN_ON_ONCE() by default, no rate limit reqd. */
>  	if ((preempt_count_equals(preempt_offset) && !irqs_disabled() &&
> @@ -7551,6 +7563,11 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
>  		return;
>  	prev_jiffy = jiffies;
>  
> +#ifdef CONFIG_DEBUG_PREEMPT
> +	/* Save this before calling printk(), since that will clobber it */
> +	preempt_disable_ip = current->preempt_disable_ip;
> +#endif
> +
>  	printk(KERN_ERR
>  		"BUG: sleeping function called from invalid context at %s:%d\n",
>  			file, line);
> @@ -7568,7 +7585,7 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
>  #ifdef CONFIG_DEBUG_PREEMPT
>  	if (!preempt_count_equals(preempt_offset)) {
>  		pr_err("Preemption disabled at:");
> -		print_ip_sym(current->preempt_disable_ip);
> +		print_ip_sym(preempt_disable_ip);
>  		pr_cont("\n");

Ditto.

Thanks,

	Ingo