linux-kernel - Re: [PATCH v6 20/29] init/main: Delay initialization of the lockup detector after smp

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Date:   Fri, 13 May 2022 16:16:40 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Andi Kleen <ak@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Stephane Eranian <eranian@...gle.com>,
        iommu@...ts.linux-foundation.org, Joerg Roedel <joro@...tes.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v6 20/29] init/main: Delay initialization of the lockup
 detector after smp_init()

On Tue, May 10, 2022 at 08:38:22PM +1000, Nicholas Piggin wrote:
> Excerpts from Ricardo Neri's message of May 6, 2022 9:59 am:
> > Certain implementations of the hardlockup detector require support for
> > Inter-Processor Interrupt shorthands. On x86, support for these can only
> > be determined after all the possible CPUs have booted once (in
> > smp_init()). Other architectures may not need such check.
> > 
> > lockup_detector_init() only performs the initializations of data
> > structures of the lockup detector. Hence, there are no dependencies on
> > smp_init().
> 

Thank you for your feedback Nicholas!

> I think this is the only real thing which affects other watchdog types?

Also patches 18 and 19 that decouple the NMI watchdog functionality from
perf.

> 
> Not sure if it's a big problem, the secondary CPUs coming up won't
> have their watchdog active until quite late, and the primary could
> implement its own timeout in __cpu_up for secondary coming up, and
> IPI it to get traces if necessary which is probably more robust.

Indeed that could work. Another alternative I have been pondering is to boot
the system with the perf-based NMI watchdog enabled. Once all CPUs are up
and running, switch to the HPET-based NMI watchdog and free the PMU counters.

> 
> Acked-by: Nicholas Piggin <npiggin@...il.com>

Thank you!

BR,
Ricardo