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: <20190618224654.GB30488@ranerica-svr.sc.intel.com>
Date:   Tue, 18 Jun 2019 15:46:54 -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>,
        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 Tue, Jun 11, 2019 at 10:11:04PM +0200, Thomas Gleixner wrote:
> 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....

Do you mean adding a readback to check if the new compare value is
greater than the current count? Similar to the check at the end of
hpet_next_event():

	return res < HPET_MIN_CYCLES ? -ETIME : 0;

In such a case, should it try to set the comparator again? I think it
should, as otherwise the hardlockup detector would stop working.

Thanks and BR,
Ricardo
> 
> Thanks,
> 
> 	tglx