linux-kernel - Re: [PATCH] softlockup: make detector be aware of task switch of processes hogging cpu

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Date:	Thu, 21 Aug 2014 21:58:22 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	chai wen <chaiw.fnst@...fujitsu.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] softlockup: make detector be aware of task switch of
 processes hogging cpu

On Thu, Aug 21, 2014 at 01:42:22PM +0800, chai wen wrote:
> For now, soft lockup detector warns once for each case of process softlockup.
> But the thread 'watchdog/n' may not always get the cpu at the time slot between
> the task switch of two processes hogging that cpu to reset soft_watchdog_warn.
> 
> An example would be two processes hogging the cpu.  Process A causes the
> softlockup warning and is killed manually by a user.  Process B immediately
> becomes the new process hogging the cpu preventing the softlockup code from
> resetting the soft_watchdog_warn variable.
> 
> This case is a false negative of "warn only once for a process", as there may
> be a different process that is going to hog the cpu.  Resolve this by
> saving/checking the task pointer of the hogging process and use that to reset
> soft_watchdog_warn too.
> 
> Signed-off-by: chai wen <chaiw.fnst@...fujitsu.com>
> Signed-off-by: Don Zickus <dzickus@...hat.com>

Acked-by: Don Zickus <dzickus@...hat.com>

> ---
>  kernel/watchdog.c |   16 +++++++++++++++-
>  1 files changed, 15 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 0037db6..2e55620 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -42,6 +42,7 @@ static DEFINE_PER_CPU(bool, softlockup_touch_sync);
>  static DEFINE_PER_CPU(bool, soft_watchdog_warn);
>  static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
>  static DEFINE_PER_CPU(unsigned long, soft_lockup_hrtimer_cnt);
> +static DEFINE_PER_CPU(struct task_struct *, softlockup_task_ptr_saved);
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>  static DEFINE_PER_CPU(bool, hard_watchdog_warn);
>  static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
> @@ -328,8 +329,20 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  			return HRTIMER_RESTART;
>  
>  		/* only warn once */
> -		if (__this_cpu_read(soft_watchdog_warn) == true)
> +		if (__this_cpu_read(soft_watchdog_warn) == true) {
> +			/*
> +			 * Handle the case where multiple processes are
> +			 * causing softlockups but the duration is small
> +			 * enough, the softlockup detector can not reset
> +			 * itself in time.  Use task pointers to detect this.
> +			 */
> +			if (__this_cpu_read(softlockup_task_ptr_saved) !=
> +			    current) {
> +				__this_cpu_write(soft_watchdog_warn, false);
> +				__touch_watchdog();
> +			}
>  			return HRTIMER_RESTART;
> +		}
>  
>  		if (softlockup_all_cpu_backtrace) {
>  			/* Prevent multiple soft-lockup reports if one cpu is already
> @@ -345,6 +358,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  		pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
>  			smp_processor_id(), duration,
>  			current->comm, task_pid_nr(current));
> +		__this_cpu_write(softlockup_task_ptr_saved, current);
>  		print_modules();
>  		print_irqtrace_events(current);
>  		if (regs)
> -- 
> 1.7.1
> 
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