linux-kernel - Re: [RFC PATCH 20/23] watchdog/hardlockup/hpet: Rotate interrupt among all monitored CPUs

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Date:   Sat, 16 Jun 2018 15:27:59 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
cc:     Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, x86@...nel.org,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, 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@...cle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Randy Dunlap <rdunlap@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Christoffer Dall <cdall@...aro.org>,
        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>,
        iommu@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 20/23] watchdog/hardlockup/hpet: Rotate interrupt
 among all monitored CPUs

On Fri, 15 Jun 2018, Ricardo Neri wrote:
> On Fri, Jun 15, 2018 at 12:29:06PM +0200, Thomas Gleixner wrote:
> > You have to consider two cases:
> > 
> >  1) !remapped mode:
> > 
> >     That's reasonably simple because you just have to deal with the HPET
> >     TIMERn_PROCMSG_ROUT register. But then you need to do this directly and
> >     not through any of the existing interrupt facilities.
> 
> Indeed, there is no need to use the generic interrupt faciities to set affinity;
> I am dealing with an NMI anyways.
> > 
> >  2) remapped mode:
> > 
> >     That's way more complex as you _cannot_ ever do anything which touches
> >     the IOMMU and the related tables.
> > 
> >     So you'd need to reserve an IOMMU remapping entry for each CPU upfront,
> >     store the resulting value for the HPET TIMERn_PROCMSG_ROUT register in
> >     per cpu storage and just modify that one from NMI.
> > 
> >     Though there might be subtle side effects involved, which are related to
> >     the acknowledge part. You need to talk to the IOMMU wizards first.
> 
> I see. I will look into the code and prototype something that makes sense for
> the IOMMU maintainers.

I'd recommend to talk to them _before_ you cobble something together. If we
cannot reliably switch the affinity by directing the HPET NMI to a
different IOMMU remapping entry then the whole scheme does not work at all.

Thanks,

	tglx