linux-kernel - Re: [RFC PATCH 1/4] watchdog: hpwdt: Don't disable watchdog on NMI

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Date:   Tue, 15 Jan 2019 18:52:01 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Jerry.Hoemann@....com, Ivan Mironov <mironov.ivan@...il.com>
Cc:     linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org,
        Wim Van Sebroeck <wim@...ux-watchdog.org>
Subject: Re: [RFC PATCH 1/4] watchdog: hpwdt: Don't disable watchdog on NMI

On 1/15/19 6:27 PM, Jerry Hoemann wrote:
> On Mon, Jan 14, 2019 at 07:36:14AM +0500, Ivan Mironov wrote:
>> Existing code disables watchdog on NMI right before completely hanging
>> the system.
>>
>> There are two problems here:
>>
>>   * First, watchdog is expected to reset the system in a case of such
>>     failure, no matter what.
> 
> Documentation/watchdog/watchdog-api.txt
> 
> explicitly allows for pretimeout NMI and generation of kernel crash dumps.
> 
> By removing hpwdt_stop the system will likely fail to crash dump
> as there is only 9 seconds between receipt of a NMI and the iLO
> resetting the system.
> 
> Unfortunately, kdump is not without issues and can also be difficult
> to properly configure either of which can result in failure to dump
> and reset.
> 
> Customers who value availability over kdump collection, the pretimeout
> NMI can be disabled and hardware will not issue the pretimeout NMI
> and will only do reset.
> 
> A middle ground for those who want tombstones but not kdump, would
> be to leave the pretimeout NMI enabled and add "panic=N" to the
> Linux command line.  That way after the panic, the tombstone is
> printed and the system resets after N seconds.
> 
> 
> 
>>   * Second, this code has no effect if there are more than one watchdog.
> 
> That is correct.  Hpwdt will not turn off any other WDT.
> 
> I don't see a current method of notifying other watchdogs
> that a given watchdog is going to take the system down.
> 
And there should not be.

> The closest I hook see is watchdog_notify_pretimeout, but I don't
> see that notifying other WDT.  Its not clear to me that it should.
> (e.g. the second WDT could be of longer duration and protect against
> kdump hanging. This would need to be thought through.)
> 

Watchdogs are independent of each other. If there is more than one,
they need to be configured carefully (just like pretimeout vs. timeout).
It is not up to the kernel to let watchdogs interfere with each other.

Guenter

> 
> 
>>
>> Signed-off-by: Ivan Mironov <mironov.ivan@...il.com>
>> ---
>>   drivers/watchdog/hpwdt.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/drivers/watchdog/hpwdt.c b/drivers/watchdog/hpwdt.c
>> index ef30c7e9728d..2467e6bc25c2 100644
>> --- a/drivers/watchdog/hpwdt.c
>> +++ b/drivers/watchdog/hpwdt.c
>> @@ -170,8 +170,6 @@ static int hpwdt_pretimeout(unsigned int ulReason, struct pt_regs *regs)
>>   	if (ilo5 && !pretimeout && !mynmi)
>>   		return NMI_DONE;
>>   
>> -	hpwdt_stop();
>> -
>>   	hex_byte_pack(panic_msg, mynmi);
>>   	nmi_panic(regs, panic_msg);
>>   
>> -- 
>> 2.20.1
>