linux-kernel - Re: [PATCH 0/2] Introduce update_arch_nmi

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Date:   Thu, 13 Oct 2016 15:37:21 -0500
From:   Babu Moger <babu.moger@...cle.com>
To:     Don Zickus <dzickus@...hat.com>
Cc:     mingo@...nel.org, akpm@...ux-foundation.org, ak@...ux.intel.com,
        jkosina@...e.cz, baiyaowei@...s.chinamobile.com,
        atomlin@...hat.com, uobergfe@...hat.com, tj@...nel.org,
        hidehiro.kawai.ez@...achi.com, johunt@...mai.com,
        davem@...emloft.net, sparclinux@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Introduce update_arch_nmi_watchdog for arch specific
 handlers


On 10/7/2016 10:51 AM, Don Zickus wrote:
> On Thu, Oct 06, 2016 at 03:16:41PM -0700, Babu Moger wrote:
>> During our testing we noticed that nmi watchdogs in sparc could not be disabled or
>> enabled dynamically using sysctl/proc interface. Sparc uses its own arch specific
>> nmi watchdogs. There is a sysctl and proc interface(proc/sys/kernel/nmi_watchdog)
>> to enable/disable nmi watchdogs. However, that is not working for sparc. There
>> is no interface to feed this parameter to arch specific nmi watchdogs.
>>
>> These patches extend the same sysctl/proc interface to enable or disable
>> these arch specific nmi watchdogs dynamically. Introduced new function
>> update_arch_nmi_watchdog which can be implemented in arch specific handlers.
>> If you think there is a better way to do this. Please advice.
>>
>> Tested on sparc. Compile tested on x86.
> Hi Babu,
>
> Thanks for the patch.  Yeah, I don't test sparc at all (lack of hardware).
> Sorry about that.
>
> We did spend quite a bit of time trying to get various soft/hard lockup
> logic going for the /proc stuff and I am wondering if your patches are to
> simple and expose some of the races we tried to fix.
>
> Therefore I am wondering if we could re-use some of our logic for your case.
>
> The perf stuff is really the x86 equivalent of arch_watchdog_enable.  I am
> wondering if we break that out as a __weak default function and then have
> sparc override it with its own enable/disable functions.  Something along
> the lines below (compiled on x86 but untested)?
Hi Don, Sorry for the late response. I ran into issues with the setups 
and new approach.
  I could not use your patches as is. Reason is sparc does not define 
CONFIG_HARDLOCKUP_DETECTOR.
  So, defining default __weak function did not  work for me. However, I 
have used your idea to
define __weak functions  arch_watchdog_nmi_enable and 
arch_watchdog_nmi_disable when
CONFIG_HARDLOCKUP_DETECTOR  is not defined. Sending v2 version now.
Please take a look. Thanks for your inputs.
>
> Cheers,
> Don
>
>
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 9acb29f..55cd2d3 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -585,15 +585,11 @@ static void watchdog(unsigned int cpu)
>    */
>   static unsigned long cpu0_err;
>   
> -static int watchdog_nmi_enable(unsigned int cpu)
> +int __weak arch_watchdog_nmi_enable(unsigned int cpu)
>   {
>   	struct perf_event_attr *wd_attr;
>   	struct perf_event *event = per_cpu(watchdog_ev, cpu);
>   
> -	/* nothing to do if the hard lockup detector is disabled */
> -	if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
> -		goto out;
> -
>   	/* is it already setup and enabled? */
>   	if (event && event->state > PERF_EVENT_STATE_OFF)
>   		goto out;
> @@ -619,18 +615,6 @@ static int watchdog_nmi_enable(unsigned int cpu)
>   		goto out_save;
>   	}
>   
> -	/*
> -	 * Disable the hard lockup detector if _any_ CPU fails to set up
> -	 * set up the hardware perf event. The watchdog() function checks
> -	 * the NMI_WATCHDOG_ENABLED bit periodically.
> -	 *
> -	 * The barriers are for syncing up watchdog_enabled across all the
> -	 * cpus, as clear_bit() does not use barriers.
> -	 */
> -	smp_mb__before_atomic();
> -	clear_bit(NMI_WATCHDOG_ENABLED_BIT, &watchdog_enabled);
> -	smp_mb__after_atomic();
> -
>   	/* skip displaying the same error again */
>   	if (cpu > 0 && (PTR_ERR(event) == cpu0_err))
>   		return PTR_ERR(event);
> @@ -658,7 +642,36 @@ out:
>   	return 0;
>   }
>   
> -static void watchdog_nmi_disable(unsigned int cpu)
> +static int watchdog_nmi_enable(unsigned int cpu)
> +{
> +	int err;
> +
> +	/* nothing to do if the hard lockup detector is disabled */
> +	if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
> +		return 0;
> +
> +	err = arch_watchdog_nmi_enable(cpu);
> +
> +	if (err) {
> +		/*
> +		 * Disable the hard lockup detector if _any_ CPU fails to set up
> +		 * set up the hardware perf event. The watchdog() function checks
> +		 * the NMI_WATCHDOG_ENABLED bit periodically.
> +		 *
> +		 * The barriers are for syncing up watchdog_enabled across all the
> +		 * cpus, as clear_bit() does not use barriers.
> +		 */
> +		smp_mb__before_atomic();
> +		clear_bit(NMI_WATCHDOG_ENABLED_BIT, &watchdog_enabled);
> +		smp_mb__after_atomic();
> +
> +		return err;
> +	}
> +
> +	return 0;
> +}
> +
> +void __weak arch_watchdog_nmi_disable(unsigned int cpu)
>   {
>   	struct perf_event *event = per_cpu(watchdog_ev, cpu);
>   
> @@ -675,6 +688,11 @@ static void watchdog_nmi_disable(unsigned int cpu)
>   	}
>   }
>   
> +static void watchdog_nmi_disable(unsigned int cpu)
> +{
> +	arch_watchdog_nmi_disable(cpu);
> +}
> +
>   #else
>   static int watchdog_nmi_enable(unsigned int cpu) { return 0; }
>   static void watchdog_nmi_disable(unsigned int cpu) { return; }