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

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Date:	Tue, 26 Aug 2014 10:22:14 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Chai Wen <chaiw.fnst@...fujitsu.com>
Cc:	akpm@...ux-foundation.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] softlockup: make detector be aware of task switch of
 processes hogging cpu

On Tue, Aug 26, 2014 at 08:51:30PM +0800, Chai Wen wrote:
> On 08/22/2014 09:58 AM, Don Zickus wrote:
> 
> > 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>
> > 
> 
> 
> Hi Andrew
> 
> Sorry for some disturbing.
> Could you help to check and pick up this little improvement patch ?
> 
> I am not sure which MAINTAINER I should talk to, but the original version of
> this patch is queued to -mm tree by you, so I assume that they are in the charge of you.
> 
> 
> thanks
> chai wen

Hi Chai,

Sorry about that.  Ingo asked me privately to pick this up and re-post
with my signoff.  I was converting to a new test env and was going to use this
patch as an excuse to exercise it.  That is the delay.  Let me get this
out today.

Cheers,
Don

> 
> >> ---
> >>  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
> >>
> > .
> > 
> 
> 
> 
> -- 
> Regards
> 
> Chai Wen
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