linux-kernel - Re: [PATCH 2/2 v4] watchdog: Printing traces for all cpus on lockup detection

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Date:	Thu, 24 Apr 2014 09:50:05 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	davem@...emloft.net, sparclinux@...r.kernel.org, mguzik@...hat.com,
	Aaron Tomlin <atomlin@...hat.com>
Subject: Re: [PATCH 2/2 v4] watchdog: Printing traces for all cpus on lockup
 detection

On Thu, Apr 24, 2014 at 09:48:04AM -0400, Don Zickus wrote:
> On Wed, Apr 23, 2014 at 02:14:07PM -0700, Andrew Morton wrote:
> > On Wed, 23 Apr 2014 16:40:05 -0400 Don Zickus <dzickus@...hat.com> wrote:
> > 
> > > From: Aaron Tomlin <atomlin@...hat.com>
> > > 
> > > A 'softlockup' is defined as a bug that causes the kernel to
> > > loop in kernel mode for more than a predefined period to
> > > time, without giving other tasks a chance to run.
> > > 
> > > Currently, upon detection of this condition by the per-cpu
> > > watchdog task, debug information (including a stack trace)
> > > is sent to the system log.
> > > 
> > > On some occasions, we have observed that the "victim" rather
> > > than the actual "culprit" (i.e. the owner/holder of the
> > > contended resource) is reported to the user.
> > > Often this information has proven to be insufficient to
> > > assist debugging efforts.
> > > 
> > > To avoid loss of useful debug information, for architectures
> > > which support NMI, this patch makes it possible to improve
> > > soft lockup reporting. This is accomplished by issuing an
> > > NMI to each cpu to obtain a stack trace.
> > > 
> > > If NMI is not supported we just revert back to the old method.
> > > A sysctl and boot-time parameter is available to toggle this
> > > feature.
> > > 
> > > --- a/include/linux/nmi.h
> > > +++ b/include/linux/nmi.h
> > > @@ -57,6 +57,9 @@ int hw_nmi_is_cpu_stuck(struct pt_regs *);
> > >  u64 hw_nmi_get_sample_period(int watchdog_thresh);
> > >  extern int watchdog_user_enabled;
> > >  extern int watchdog_thresh;
> > > +#ifdef CONFIG_SMP
> > > +extern int sysctl_softlockup_all_cpu_backtrace;
> > > +#endif
> > 
> > The ifdefs aren't really needed here.  If we omit them then error
> > reporting happens at link time rather than at compile time, but that's
> > a small price to pay for cleaning up the code.
> > 
> > > +		if (softlockup_all_cpu_backtrace) {
> > > +			/* Prevent multiple soft-lockup reports if one cpu is already
> > > +			 * engaged in dumping cpu back traces
> > > +			 */
> > > +			if (test_and_set_bit(0, &soft_lockup_nmi_warn)) {
> > > +				/* Someone else will report us. Let's give up */
> > > +				__this_cpu_write(soft_watchdog_warn, true);
> > > +				return HRTIMER_RESTART;
> > > +			}
> > > +		}
> > 
> > You missed my suggestion here.
> > 
> >    text    data     bss     dec     hex filename
> >    1519     524      24    2067     813 kernel/watchdog.o-before
> >    1471     520      16    2007     7d7 kernel/watchdog.o-after
> > 
> > 
> > --- a/include/linux/nmi.h~watchdog-printing-traces-for-all-cpus-on-lockup-detection-fix
> > +++ a/include/linux/nmi.h
> > @@ -57,9 +57,7 @@ int hw_nmi_is_cpu_stuck(struct pt_regs *
> >  u64 hw_nmi_get_sample_period(int watchdog_thresh);
> >  extern int watchdog_user_enabled;
> >  extern int watchdog_thresh;
> > -#ifdef CONFIG_SMP
> >  extern int sysctl_softlockup_all_cpu_backtrace;
> > -#endif
> >  struct ctl_table;
> >  extern int proc_dowatchdog(struct ctl_table *, int ,
> >  			   void __user *, size_t *, loff_t *);
> > --- a/kernel/watchdog.c~watchdog-printing-traces-for-all-cpus-on-lockup-detection-fix
> > +++ a/kernel/watchdog.c
> > @@ -31,7 +31,12 @@
> >  
> >  int watchdog_user_enabled = 1;
> >  int __read_mostly watchdog_thresh = 10;
> > +#ifdef CONFIG_SMP
> >  int __read_mostly sysctl_softlockup_all_cpu_backtrace;
> > +#else
> > +#define sysctl_softlockup_all_cpu_backtrace 0
> > +#endif
> > +
> >  static int __read_mostly watchdog_running;
> >  static u64 __read_mostly sample_period;
> >  
> > _
> 
> Ah ok.  I will respin the patch with that cleanup.  Thanks!

Or I can just be happy you took care of that for me.  :-)

/me should read all his email first...

Thanks,
Don
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