linux-kernel - Re: [PATCH] hardlockup: detect hard lockups without NMIs using secondary cpus

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Date:	Thu, 10 Jan 2013 21:34:00 -0800
From:	Colin Cross <ccross@...roid.com>
To:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] hardlockup: detect hard lockups without NMIs using
 secondary cpus

On Thu, Jan 10, 2013 at 5:39 PM, Liu, Chuansheng
<chuansheng.liu@...el.com> wrote:
>
>
>> -----Original Message-----
>> From: Colin Cross [mailto:ccross@...roid.com]
>> Sent: Thursday, January 10, 2013 9:58 AM
>> To: linux-kernel@...r.kernel.org
>> Cc: Andrew Morton; Don Zickus; Ingo Molnar; Thomas Gleixner; Liu,
>> Chuansheng; linux-arm-kernel@...ts.infradead.org; Colin Cross
>> Subject: [PATCH] hardlockup: detect hard lockups without NMIs using
>> secondary cpus
>>
>> Emulate NMIs on systems where they are not available by using timer
>> interrupts on other cpus.  Each cpu will use its softlockup hrtimer
>> to check that the next cpu is processing hrtimer interrupts by
>> verifying that a counter is increasing.
>>
>> This patch is useful on systems where the hardlockup detector is not
>> available due to a lack of NMIs, for example most ARM SoCs.
>> Without this patch any cpu stuck with interrupts disabled can
>> cause a hardware watchdog reset with no debugging information,
>> but with this patch the kernel can detect the lockup and panic,
>> which can result in useful debugging info.
>>
>> Signed-off-by: Colin Cross <ccross@...roid.com>
>> +static void watchdog_check_hardlockup_other_cpu(void)
>> +{
>> +     int cpu;
>> +     cpumask_t cpus = watchdog_cpus;
>> +
>> +     /*
>> +      * Test for hardlockups every 3 samples.  The sample period is
>> +      *  watchdog_thresh * 2 / 5, so 3 samples gets us back to slightly over
>> +      *  watchdog_thresh (over by 20%).
>> +      */
>> +     if (__this_cpu_read(hrtimer_interrupts) % 3 != 0)
>> +             return;
>> +
>> +     /* check for a hardlockup on the next cpu */
>> +     cpu = cpumask_next(smp_processor_id(), &cpus);
>> +     if (cpu >= nr_cpu_ids)
>> +             cpu = cpumask_first(&cpus);
>> +     if (cpu == smp_processor_id())
>> +             return;
>> +
>> +     smp_rmb();
>> +
>> +     if (per_cpu(watchdog_nmi_touch, cpu) == true) {
>> +             per_cpu(watchdog_nmi_touch, cpu) = false;
>> +             return;
>> +     }
>> +
>> +     if (is_hardlockup_other_cpu(cpu)) {
>> +             /* only warn once */
> One possible case for new hotplug CPU that false hardlockup case.
> 1/ Assume CPU1, CPU2 are online, CPU3 is being hotplug:
> CPU3:                                            CPU2:
> watchdog_nmi_enable()
>  per_cpu(watchdog_nmi_touch, cpu) = true;
>  cpumask_set_cpu(cpu, &watchdog_cpus);
>                                                  watchdog_check_hardlockup_other_cpu()
>                                                    per_cpu(watchdog_nmi_touch, cpu) = false; == > Here cpu is CPU3
>
> 2/ Before CPU3's first hrtimer interrupt coming, CPU2 is been offlined.
>   Then CPU1's next CPU is CPU3. But we can not use CPU3's watchdog_nmi_touch to defense
>   false CPU3 hardlock more. When CPU1's hrtimer interrupt coming, it is possible report CPU3
>   false hard lockup.
>
> Is it the case?

Yes, this is the same race condition I pointed out in reply to Don
Zickus earlier in the thread.  I think the easiest solution is to set
per_cpu(watchdog_nmi_touch, next_cpu) = true during cpu offlining.
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