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Date:   Tue, 9 Apr 2019 12:59:46 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
        Ashok Raj <ashok.raj@intel.com>,
        Andi Kleen <andi.kleen@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Ricardo Neri <ricardo.neri@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Tony Luck <tony.luck@intel.com>,
        Clemens Ladisch <clemens@ladisch.de>,
        Arnd Bergmann <arnd@arndb.de>,
        Philippe Ombredanne <pombredanne@nexb.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Jan Kiszka <jan.kiszka@siemens.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Nayna Jain <nayna@linux.ibm.com>
Subject: Re: [RFC PATCH v2 11/14] x86/watchdog/hardlockup: Add an HPET-based
 hardlockup detector
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On Tue, Mar 26, 2019 at 09:49:13PM +0100, Thomas Gleixner wrote:
> So way you should handle this is:
> 
> 	cpumask_set_cpu(cpu, hld_data->cpu_monitored_mask);
> 
> 	if (!hld_data->enabled_cpus++) {
> 		hld_data->handling_cpu = cpu;
> 		kick_timer();
> 		enable_timer();
> 	}
> 
> The cpu mask starts off empty and each CPU sets itself when the function is
> invoked on it.
> 
> data->enabled_cpus keeps track of the enabled cpus so you avoid
> reconfiguration just because a different cpu comes online. And it's
> required for disable as well.
> 
> > +void hardlockup_detector_hpet_disable(void)
> > +{
> > +	struct cpumask *allowed = watchdog_get_allowed_cpumask();
> > +
> > +	if (!hld_data)
> > +		return;
> > +
> > +	/* Only disable the timer if there are no more CPUs to monitor. */
> > +	if (!cpumask_weight(allowed))
> > +		disable_timer(hld_data);
> 
> Again this should be:
> 
> 	cpumask_clear_cpu(cpu, hld_data->cpu_monitored_mask);
> 	hld_data->enabled_cpus--;
> 
> 	if (hld_data->handling_cpu != cpu)
> 		return;
> 
> 	disable_timer();
> 	if (hld_data->enabled_cpus)
> 		return;

	if (!hld_data->enabled_cpus)
		return;

> 
> 	hld_data->handling_cpu = cpumask_first(hld_data->cpu_monitored_mask);
> 	enable_timer();

That said; you can do the above without ->enabled_cpus, by using
->handling_cpu == nr_cpu_ids to indicate 'empty'. But I'm not at all
sure that is worth the effort, it results in less obious code.