Re: [PATCH v5 2/2] watchdog: add watchdog_exclude sysctl to assist nohz

[Frederic Weisbecker]

On Mon, Apr 06, 2015 at 03:45:56PM -0400, cmetcalf@xxxxxxxxxx wrote:
> From: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> 
> Change the default behavior of watchdog so it only runs on the
> housekeeping cores when nohz_full is enabled at build and boot time.
> 
> Allow modifying the set of cores the watchdog is currently running
> on with a new kernel.watchdog_exclude sysctl.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> ---
>  Documentation/lockup-watchdogs.txt |  6 ++++++
>  Documentation/sysctl/kernel.txt    |  9 +++++++++
>  include/linux/nmi.h                |  3 +++
>  kernel/sysctl.c                    |  7 +++++++
>  kernel/watchdog.c                  | 33 +++++++++++++++++++++++++++++++++
>  5 files changed, 58 insertions(+)
> 
> diff --git a/Documentation/lockup-watchdogs.txt b/Documentation/lockup-watchdogs.txt
> index ab0baa692c13..4f86aec1d69d 100644
> --- a/Documentation/lockup-watchdogs.txt
> +++ b/Documentation/lockup-watchdogs.txt
> @@ -61,3 +61,9 @@ As explained above, a kernel knob is provided that allows
>  administrators to configure the period of the hrtimer and the perf
>  event. The right value for a particular environment is a trade-off
>  between fast response to lockups and detection overhead.
> +
> +By default, the watchdog runs on all online cores.  However, on a
> +kernel configured with NO_HZ_FULL, by default the watchdog runs only
> +on the housekeeping cores, not the cores specified in the "nohz_full"
> +boot argument.  In either case, the set of cores excluded from running
> +the watchdog may be adjusted via the kernel.watchdog_exclude sysctl.
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index c831001c45f1..799a1fee3f26 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -923,6 +923,15 @@ and nmi_watchdog.
>  
>  ==============================================================
>  
> +watchdog_exclude:
> +
> +This value can be used to control on which cpus the watchdog is
> +prohibited from running.  The default exclude mask is empty, but if
> +NO_HZ_FULL is enabled in the kernel config, and cores are specified
> +with the nohz_full= boot argument, those cores are excluded by default.
> +
> +==============================================================

Same here, cpumask are rather used as inclusive instead of exclusive.
So I'd rather see "watchdog_cpumask".

> +
>  watchdog_thresh:
>  
>  This value can be used to control the frequency of hrtimer and NMI
> diff --git a/include/linux/nmi.h b/include/linux/nmi.h
> index 3d46fb4708e0..5094386b4fb1 100644
> --- a/include/linux/nmi.h
> +++ b/include/linux/nmi.h
> @@ -67,6 +67,7 @@ extern int nmi_watchdog_enabled;
>  extern int soft_watchdog_enabled;
>  extern int watchdog_user_enabled;
>  extern int watchdog_thresh;
> +extern unsigned long *watchdog_exclude_mask_bits;
>  extern int sysctl_softlockup_all_cpu_backtrace;
>  struct ctl_table;
>  extern int proc_watchdog(struct ctl_table *, int ,
> @@ -77,6 +78,8 @@ extern int proc_soft_watchdog(struct ctl_table *, int ,
>  			      void __user *, size_t *, loff_t *);
>  extern int proc_watchdog_thresh(struct ctl_table *, int ,
>  				void __user *, size_t *, loff_t *);
> +extern int proc_watchdog_exclude(struct ctl_table *, int,
> +				 void __user *, size_t *, loff_t *);
>  #endif
>  
>  #ifdef CONFIG_HAVE_ACPI_APEI_NMI
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 2082b1a88fb9..b934a4a01f0a 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -881,6 +881,13 @@ static struct ctl_table kern_table[] = {
>  		.extra2		= &one,
>  	},
>  	{
> +		.procname	= "watchdog_exclude",
> +		.data		= &watchdog_exclude_mask_bits,
> +		.maxlen		= NR_CPUS,
> +		.mode		= 0644,
> +		.proc_handler	= proc_watchdog_exclude,
> +	},
> +	{
>  		.procname	= "softlockup_panic",
>  		.data		= &softlockup_panic,
>  		.maxlen		= sizeof(int),
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index f2be11ab7e08..3f4fbb208437 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -19,6 +19,7 @@
>  #include <linux/sysctl.h>
>  #include <linux/smpboot.h>
>  #include <linux/sched/rt.h>
> +#include <linux/tick.h>
>  
>  #include <asm/irq_regs.h>
>  #include <linux/kvm_para.h>
> @@ -56,6 +57,8 @@ int __read_mostly sysctl_softlockup_all_cpu_backtrace;
>  #else
>  #define sysctl_softlockup_all_cpu_backtrace 0
>  #endif
> +static cpumask_var_t watchdog_exclude_mask;
> +unsigned long *watchdog_exclude_mask_bits;
>  
>  static int __read_mostly watchdog_running;
>  static u64 __read_mostly sample_period;
> @@ -841,12 +844,42 @@ out:
>  	mutex_unlock(&watchdog_proc_mutex);
>  	return err;
>  }
> +
> +int proc_watchdog_exclude(struct ctl_table *table, int write,
> +			  void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	int err;
> +
> +	mutex_lock(&watchdog_proc_mutex);
> +	err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
> +	if (!err && write && watchdog_enabled && watchdog_thresh) {
> +		watchdog_disable_all_cpus();
> +		watchdog_enable_all_cpus();

The problem is that it modifies watchdog_threads.exclude_mask and watchdog_threads
is a live object handled by smpboot. It happens not to be racy now because smpboot
only checks this cpumask on per_cpu_threads register/unregister time, but it could
change and become racy in the future.

How about creating smpboot_update_mask_percpu_thread() and handle it from smpboot,
this way future evolutions of smpboot won't overlook this cpumask live change?

Thanks.

> +	}
> +	mutex_unlock(&watchdog_proc_mutex);
> +	return err;
> +}
> +
>  #endif /* CONFIG_SYSCTL */
>  
>  void __init lockup_detector_init(void)
>  {
>  	set_sample_period();
>  
> +	alloc_bootmem_cpumask_var(&watchdog_exclude_mask);
> +	watchdog_threads.exclude_mask = watchdog_exclude_mask;
> +
> +#ifdef CONFIG_NO_HZ_FULL
> +	if (!cpumask_empty(tick_nohz_full_mask))
> +		pr_info("Disabling watchdog on nohz_full cores by default\n");
> +	cpumask_copy(watchdog_exclude_mask, tick_nohz_full_mask);
> +#else
> +	cpumask_clear(watchdog_exclude_mask);
> +#endif
> +
> +	/* The sysctl API requires a variable holding a pointer to the mask. */
> +	watchdog_exclude_mask_bits = cpumask_bits(watchdog_exclude_mask);
> +
>  	if (watchdog_enabled)
>  		watchdog_enable_all_cpus();
>  }
> -- 
> 2.1.2
> 
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