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From:   Joel Fernandes <joelaf@google.com>
Date:   Tue, 01 May 2018 18:18:44 +0000
Message-ID: <CAJWu+oqH0-nwBNSXiuuf1gPK62rstSYBaus89dmanoCw1QYv7A@mail.gmail.com>
Subject: Re: [PATCH v2 0/6] arm64: provide pseudo NMI with GICv3
To:     Daniel Thompson <daniel.thompson@linaro.org>
Cc:     julien.thierry@arm.com,
        "Joel Fernandes (Google)" <joel.opensrc@gmail.com>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <marc.zyngier@arm.com>,
        James Morse <james.morse@arm.com>
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> > On 29/04/18 07:37, Joel Fernandes wrote:
> > > On Wed, Jan 17, 2018 at 4:10 AM, Julien Thierry <
julien.thierry@arm.com> wrote:
> > > > Hi,
> > > >
> > > > On 17/01/18 11:54, Julien Thierry wrote:
> > > > >
> > > > > This series is a continuation of the work started by Daniel [1].
The goal
> > > > > is to use GICv3 interrupt priorities to simulate an NMI.
> > > > >
> > > >
> > > >
> > > > I have submitted a separate series making use of this feature for
the ARM
> > > > PMUv3 interrupt [1].
> > >
> > > I guess the hard lockup detector using NMI could be a nice next step
> > > to see how well it works with lock up detection. That's the main
> > > usecase for my interest. However, perf profiling is also a strong one.
> > >
> >
> > From my understanding, Linux's hardlockup detector already uses the ARM
PMU
> > interrupt to check whether some task is stuck. I haven't looked at the
> > details of the implementation yet, but in theory having the PMU
interrupt as
> > NMI should make the hard lockup detector use the NMI.
> >
> > When I do the v3, I'll have a look at this to check whether the
hardlockup
> > detector works fine when using NMI.

> That's what I saw on arch/arm (with some of the much older FIQ work).

> Once you have PMU and the appropriate config to *admit* to supporting
> hard lockup then it will "just work" and be setup automatically during
> kernel boot.

> Actually the problem then becomes that if you want to use the PMU
> for anything else then you may end up having to disable the hard
> lockup detector.

This problem is not anything pseudo-NMI specific though right?
Contention/constraints on PMU resources should be a problem even on
platforms with real NMI.

thanks,

- Joel