linux-kernel - Re: [PATCH] kernel/watchdog.c: Do not hardcode CPU 0 as the initial thread

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Date:   Tue, 03 Jan 2017 16:19:50 -0500
From:   Prarit Bhargava <prarit@...hat.com>
To:     Don Zickus <dzickus@...hat.com>
CC:     linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Tejun Heo <tj@...nel.org>,
        Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Joshua Hunt <johunt@...mai.com>,
        Ingo Molnar <mingo@...nel.org>,
        Babu Moger <babu.moger@...cle.com>
Subject: Re: [PATCH] kernel/watchdog.c: Do not hardcode CPU 0 as the initial
 thread



On 12/01/2016 03:06 PM, Don Zickus wrote:
> On Tue, Nov 29, 2016 at 08:15:21AM -0500, Prarit Bhargava wrote:
>> When CONFIG_BOOTPARAM_HOTPLUG_CPU0 is enabled, the socket containing the
>> boot cpu can be replaced.  During the hot add event, the message
>>
>> NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
>>
>> is output implying that the NMI watchdog was disabled at some point.  This
>> is not the case and the message has caused confusion for users of systems
>> that support the removal of the boot cpu socket.
>>
>> The watchdog code is coded to assume that cpu 0 is always the first cpu to
>> initialize the watchdog, and the last to stop its watchdog thread.  That
>> is not the case for initializing if cpu 0 has been removed and added.  The
>> removal case has never been correct because the smpboot code will remove
>> the watchdog threads starting with the lowest cpu number.
>>
>> This patch adds watchdog_cpus to track the number of cpus with active NMI
>> watchdog threads so that the first and last thread can be used to set and
>> clear the value of firstcpu_err.  firstcpu_err is set when the first
>> watchdog thread is enabled, and cleared when the last watchdog thread is
>> disabled.
>>
>> This patch is based on top of linux-next akpm-base.
> 
> It passed my tests.  Thanks!
> 
> Acked-by: Don Zickus <dzickus@...hat.com>

Just re-pinging on this.  I haven't seen it picked up by anyone.

P.

> 
> 
>>
>> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
>> Cc: Borislav Petkov <bp@...e.de>
>> Cc: Tejun Heo <tj@...nel.org>
>> Cc: Don Zickus <dzickus@...hat.com>
>> Cc: Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Andi Kleen <ak@...ux.intel.com>
>> Cc: Joshua Hunt <johunt@...mai.com>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: Babu Moger <babu.moger@...cle.com>
>> ---
>>  kernel/watchdog_hld.c |   25 +++++++++++++++----------
>>  1 file changed, 15 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
>> index 84016c8aee6b..30761f7504ef 100644
>> --- a/kernel/watchdog_hld.c
>> +++ b/kernel/watchdog_hld.c
>> @@ -134,12 +134,14 @@ static void watchdog_overflow_callback(struct perf_event *event,
>>   * Reduce the watchdog noise by only printing messages
>>   * that are different from what cpu0 displayed.
>>   */
>> -static unsigned long cpu0_err;
>> +static unsigned long firstcpu_err;
>> +static atomic_t watchdog_cpus;
>>  
>>  int watchdog_nmi_enable(unsigned int cpu)
>>  {
>>  	struct perf_event_attr *wd_attr;
>>  	struct perf_event *event = per_cpu(watchdog_ev, cpu);
>> +	int firstcpu = 0;
>>  
>>  	/* nothing to do if the hard lockup detector is disabled */
>>  	if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
>> @@ -153,19 +155,22 @@ int watchdog_nmi_enable(unsigned int cpu)
>>  	if (event != NULL)
>>  		goto out_enable;
>>  
>> +	if (atomic_inc_return(&watchdog_cpus) == 1)
>> +		firstcpu = 1;
>> +
>>  	wd_attr = &wd_hw_attr;
>>  	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
>>  
>>  	/* Try to register using hardware perf events */
>>  	event = perf_event_create_kernel_counter(wd_attr, cpu, NULL, watchdog_overflow_callback, NULL);
>>  
>> -	/* save cpu0 error for future comparision */
>> -	if (cpu == 0 && IS_ERR(event))
>> -		cpu0_err = PTR_ERR(event);
>> +	/* save the first cpu's error for future comparision */
>> +	if (firstcpu && IS_ERR(event))
>> +		firstcpu_err = PTR_ERR(event);
>>  
>>  	if (!IS_ERR(event)) {
>> -		/* only print for cpu0 or different than cpu0 */
>> -		if (cpu == 0 || cpu0_err)
>> +		/* only print for the first cpu initialized */
>> +		if (firstcpu || firstcpu_err)
>>  			pr_info("enabled on all CPUs, permanently consumes one hw-PMU counter.\n");
>>  		goto out_save;
>>  	}
>> @@ -183,7 +188,7 @@ int watchdog_nmi_enable(unsigned int cpu)
>>  	smp_mb__after_atomic();
>>  
>>  	/* skip displaying the same error again */
>> -	if (cpu > 0 && (PTR_ERR(event) == cpu0_err))
>> +	if (!firstcpu && (PTR_ERR(event) == firstcpu_err))
>>  		return PTR_ERR(event);
>>  
>>  	/* vary the KERN level based on the returned errno */
>> @@ -219,9 +224,9 @@ void watchdog_nmi_disable(unsigned int cpu)
>>  
>>  		/* should be in cleanup, but blocks oprofile */
>>  		perf_event_release_kernel(event);
>> -	}
>> -	if (cpu == 0) {
>> +
>>  		/* watchdog_nmi_enable() expects this to be zero initially. */
>> -		cpu0_err = 0;
>> +		if (atomic_dec_and_test(&watchdog_cpus))
>> +			firstcpu_err = 0;
>>  	}
>>  }
>> -- 
>> 1.7.9.3
>>