linux-kernel - Re: [RFC PATCH v4 12/21] watchdog/hardlockup/hpet: Adjust timer expiration on the number of monitored CPUs

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Message-ID: <alpine.DEB.2.21.1906112205170.2214@nanos.tec.linutronix.de>
Date:   Tue, 11 Jun 2019 22:11:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Ashok Raj <ashok.raj@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <andi.kleen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        Stephane Eranian <eranian@...gle.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Ricardo Neri <ricardo.neri@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Jacob Pan <jacob.jun.pan@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Don Zickus <dzickus@...hat.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Frederic Weisbecker <frederic@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Babu Moger <Babu.Moger@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Colin Ian King <colin.king@...onical.com>,
        Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC PATCH v4 12/21] watchdog/hardlockup/hpet: Adjust timer
 expiration on the number of monitored CPUs

On Thu, 23 May 2019, Ricardo Neri wrote:
> @@ -52,10 +59,10 @@ static void kick_timer(struct hpet_hld_data *hdata, bool force)
>  		return;
>  
>  	if (hdata->has_periodic)
> -		period = watchdog_thresh * hdata->ticks_per_second;
> +		period = watchdog_thresh * hdata->ticks_per_cpu;
>  
>  	count = hpet_readl(HPET_COUNTER);
> -	new_compare = count + watchdog_thresh * hdata->ticks_per_second;
> +	new_compare = count + watchdog_thresh * hdata->ticks_per_cpu;
>  	hpet_set_comparator(hdata->num, (u32)new_compare, (u32)period);

So with this you might get close to the point where you trip over the SMI
induced madness where CPUs vanish for several milliseconds in some value
add code. You really want to do a read back of the hpet to detect that. See
the comment in the hpet code. RHEL 7/8 allow up to 768 logical CPUs....

Thanks,

	tglx