linux-kernel - Re: [RFC PATCH v2 13/14] watchdog/hardlockup/hpet: Only enable the HPET watchdog via a boot parameter

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Date:   Mon, 8 Apr 2019 19:07:28 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Ashok Raj <ashok.raj@...el.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Ricardo Neri <ricardo.neri@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Clemens Ladisch <clemens@...isch.de>,
        Arnd Bergmann <arnd@...db.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Nayna Jain <nayna@...ux.ibm.com>
Subject: Re: [RFC PATCH v2 13/14] watchdog/hardlockup/hpet: Only enable the
 HPET watchdog via a boot parameter

On Tue, Mar 26, 2019 at 10:29:52PM +0100, Thomas Gleixner wrote:
> On Wed, 27 Feb 2019, Ricardo Neri wrote:
> > +			When hpet is specified, the NMI watchdog will be driven
> > +			by an HPET timer, if available in the system. Otherwise,
> > +			the perf-based implementation will be used. Specifying
> > +			hpet implies that nmi_watchdog is on.
> 
> How so?
> 
I meant to say that the user does not need to provide nmi_watchdog=1 and
nmi_watchdog=hpet separately.

I think this is true because watchdog_user_enabled in kernel/watchdog.c is set
to 1 when CONFIG_HARDLOCKUP_DETECTOR is selected. Also, if nmi_watchdog_available
is set to true if watchdog_nmi_probe() is successful.

Perhaps I can add a warning in case nmi_watchdog=hpet and either
CONFIG_HARDLOCKUP_DETECTOR or CONFIG_HARDLOCKUP_DETECTOR_HPET are not
selected?

> > +static int __init hardlockup_detector_hpet_setup(char *str)
> > +{
> > +	if (strstr(str, "hpet"))
> > +		hardlockup_use_hpet = true;
> 
> strstr()? Not really.

Is strncmp(str, "hpet", 5) more acceptable?

> 
> > +
> > +	return 0;
> > +}
> > +__setup("nmi_watchdog=", hardlockup_detector_hpet_setup);
> > +
> >  /**
> >   * hardlockup_detector_hpet_init() - Initialize the hardlockup detector
> >   *
> > @@ -405,6 +422,9 @@ int __init hardlockup_detector_hpet_init(void)
> >  {
> >  	int ret;
> >  
> > +	if (!hardlockup_use_hpet)
> > +		return -ENODEV;
> 
> This should have been there in the patch which introduces
> hardlockup_detector_hpet_init(). And this patch merily adds the command
> line magic which sets that flag.

Sure, I will move this check into the patch that introduces
hardlockup_detector_hpet_init().

> 
> > +
> >  	if (!is_hpet_enabled())
> >  		return -ENODEV;
> >  
> > diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> > index 367aa81294ef..28cad7310378 100644
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -78,7 +78,7 @@ static int __init hardlockup_panic_setup(char *str)
> >  		nmi_watchdog_user_enabled = 0;
> >  	else if (!strncmp(str, "1", 1))
> >  		nmi_watchdog_user_enabled = 1;
> > -	return 1;
> > +	return 0;
> 
> Why?

My understanding is that this is needed so that other __setup functions that also
want to check "nmi_watchdog" are able to do it. Is this understanding
not correct?

Thanks and BR,
Ricardo