Date:   Wed, 13 Sep 2017 14:06:42 -0400
From:   Don Zickus <dzickus@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Nicholas Piggin <npiggin@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Ulrich Obergfell <uobergfe@...hat.com>
Subject: Re: [patch V2 00/29] lockup_detector: Cure hotplug deadlocks and
 replace duct tape

On Tue, Sep 12, 2017 at 09:36:54PM +0200, Thomas Gleixner wrote:
> The lockup detector is broken is several ways:
> 
>     - It's deadlock prone vs. CPU hotplug in various ways. Some of these
>       are due to recursive cpus_read_lock() others are due to
>       cpus_read_lock() from CPU hotplug callbacks which immediately lock
>       the machine because cpus are write locked.
> 
>     - The handling of the cpu hotplug threads happens sideways to the
>       smpboot thread infrastructure, which is racy and pointless
> 
>     - The handling of the user space sysctl interface is a complete
>       trainwreck as it fiddles directly with variables which can be
>       modified or evaluated by the running watchdogs.
> 
>     - The perf event initialization is a trainwreck as it tries to create
>       perf events over and over even if perf is not functional (no
>       hardware, ....). To avoid excessive dmesg spam it contains magic
>       printk ratelimiting along with either wrong or useless messages.
> 
>     - The code structure is horrible as ifdef sections are scattered all
>       over the place which makes it unreadable
> 
>     - There is more wreckage, but see the changelogs for the ugly details.
>

Aside from the simple compile issue in patch 25.  I have no issues with this
patchset.  Thanks Thomas!

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

 
> The following series sanitizes the facility and addresses the problems.
> 
> Changes since V1:
> 
>     - Wrapped the perf specific calls into the weak watchdog_nmi_*
>       functions
> 
>     - Fixed the compile error pointed out by Don
> 
>     - Fixed the reconfiguration parameter inconsistency which broke
>       powerpc
> 
>     - Picked up the updated version of patch 11/29
> 
> Delta patch below.
> 
> Thanks,
> 
>         tglx
> ---
> Diffstat for the series:
> 
>  arch/parisc/kernel/process.c   |    2 
>  arch/powerpc/kernel/watchdog.c |   22 -
>  arch/x86/events/intel/core.c   |   11 
>  include/linux/nmi.h            |  121 +++----
>  include/linux/smpboot.h        |    4 
>  kernel/cpu.c                   |    6 
>  kernel/smpboot.c               |   22 -
>  kernel/sysctl.c                |   22 -
>  kernel/watchdog.c              |  633 ++++++++++++++---------------------------
>  kernel/watchdog_hld.c          |  193 ++++++------
>  10 files changed, 434 insertions(+), 602 deletions(-)
> 
> Delta patch vs. V1
> 8<------------------------
> --- a/arch/powerpc/kernel/watchdog.c
> +++ b/arch/powerpc/kernel/watchdog.c
> @@ -355,12 +355,12 @@ static void watchdog_calc_timeouts(void)
>  	wd_timer_period_ms = watchdog_thresh * 1000 * 2 / 5;
>  }
>  
> -void watchdog_nmi_reconfigure(bool stop)
> +void watchdog_nmi_reconfigure(bool run)
>  {
>  	int cpu;
>  
>  	cpus_read_lock();
> -	if (stop) {
> +	if (!run) {
>  		for_each_cpu(cpu, &wd_cpus_enabled)
>  			stop_wd_on_cpu(cpu);
>  	} else {
> --- a/include/linux/nmi.h
> +++ b/include/linux/nmi.h
> @@ -102,7 +102,7 @@ static inline void hardlockup_detector_p
>  static inline void hardlockup_detector_perf_enable(void) { }
>  static inline void hardlockup_detector_perf_cleanup(void) { }
>  # if !defined(CONFIG_HAVE_NMI_WATCHDOG)
> -static int hardlockup_detector_perf_init(void) { return -ENODEV; }
> +static inline int hardlockup_detector_perf_init(void) { return -ENODEV; }
>  static inline void arch_touch_nmi_watchdog(void) {}
>  # else
>  static int hardlockup_detector_perf_init(void) { return 0; }
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -105,18 +105,32 @@ static int __init hardlockup_all_cpu_bac
>   * softlockup watchdog threads start and stop. The arch must select the
>   * SOFTLOCKUP_DETECTOR Kconfig.
>   */
> -int __weak watchdog_nmi_enable(unsigned int cpu) { return 0; }
> -void __weak watchdog_nmi_disable(unsigned int cpu) { }
> +int __weak watchdog_nmi_enable(unsigned int cpu)
> +{
> +	hardlockup_detector_perf_enable();
> +	return 0;
> +}
> +
> +void __weak watchdog_nmi_disable(unsigned int cpu)
> +{
> +	hardlockup_detector_perf_disable();
> +}
> +
> +/* Return 0, if a NMI watchdog is available. Error code otherwise */
> +int __weak __init void watchdog_nmi_probe(void)
> +{
> +	return hardlockup_detector_perf_init();
> +}
>  
>  /**
>   * watchdog_nmi_reconfigure - Optional function to reconfigure NMI watchdogs
> - * @stop:	If true stop the watchdogs on all enabled CPUs
> - *		If false start the watchdogs on all enabled CPUs
> + * @run:	If false stop the watchdogs on all enabled CPUs
> + *		If true start the watchdogs on all enabled CPUs
>   *
>   * The core call order is:
> - * watchdog_nmi_reconfigure(true);
> - * update_variables();
>   * watchdog_nmi_reconfigure(false);
> + * update_variables();
> + * watchdog_nmi_reconfigure(true);
>   *
>   * The second call which starts the watchdogs again guarantees that the
>   * following variables are stable across the call.
> @@ -126,13 +140,13 @@ void __weak watchdog_nmi_disable(unsigne
>   *
>   * After the call the variables can be changed again.
>   */
> -void __weak watchdog_nmi_reconfigure(bool stop) { }
> +void __weak watchdog_nmi_reconfigure(bool run) { }
>  
>  /**
>   * lockup_detector_update_enable - Update the sysctl enable bit
>   *
>   * Caller needs to make sure that the NMI/perf watchdogs are off, so this
> - * can't race with hardlockup_detector_disable().
> + * can't race with watchdog_nmi_disable().
>   */
>  static void lockup_detector_update_enable(void)
>  {
> @@ -453,8 +467,7 @@ static void watchdog_enable(unsigned int
>  	__touch_watchdog();
>  	/* Enable the perf event */
>  	if (watchdog_enabled & NMI_WATCHDOG_ENABLED)
> -		hardlockup_detector_perf_enable();
> -	watchdog_nmi_enable(cpu);
> +		watchdog_nmi_enable(cpu);
>  
>  	watchdog_set_prio(SCHED_FIFO, MAX_RT_PRIO - 1);
>  }
> @@ -469,7 +482,6 @@ static void watchdog_disable(unsigned in
>  	 * between disabling the timer and disabling the perf event causes
>  	 * the perf NMI to detect a false positive.
>  	 */
> -	hardlockup_detector_perf_disable();
>  	watchdog_nmi_disable(cpu);
>  	hrtimer_cancel(hrtimer);
>  }
> @@ -745,12 +757,6 @@ int proc_watchdog_cpumask(struct ctl_tab
>  }
>  #endif /* CONFIG_SYSCTL */
>  
> -static __init void detect_nmi_watchdog(void)
> -{
> -	if (!hardlockup_detector_perf_init())
> -		nmi_watchdog_available = true;
> -}
> -
>  void __init lockup_detector_init(void)
>  {
>  #ifdef CONFIG_NO_HZ_FULL
> @@ -763,6 +769,7 @@ void __init lockup_detector_init(void)
>  	cpumask_copy(&watchdog_cpumask, cpu_possible_mask);
>  #endif
>  
> -	detect_nmi_watchdog();
> +	if (!watchdog_nmi_probe())
> +		nmi_watchdog_available = true;
>  	softlockup_init_threads();
>  }
> 
> 
> 

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