linux-kernel - Re: [PATCH v11 4/6] arm64: smp: Add arch support for backtrace using pseudo-NMI

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Message-ID: <CAD=FV=U24kF7WbZPKMbk=kJMU14+yFJXpGWWfj0fPbA9J6h0Eg@mail.gmail.com>
Date:   Fri, 25 Aug 2023 16:02:46 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sumit Garg <sumit.garg@...aro.org>,
        Will Deacon <will@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kgdb-bugreport@...ts.sourceforge.net,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        linux-perf-users@...r.kernel.org,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Chen-Yu Tsai <wens@...e.org>,
        linux-arm-kernel@...ts.infradead.org, ito-yuichi@...itsu.com,
        Peter Zijlstra <peterz@...radead.org>,
        D Scott Phillips <scott@...amperecomputing.com>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 4/6] arm64: smp: Add arch support for backtrace using pseudo-NMI

Hi,

On Fri, Aug 25, 2023 at 3:27 PM Stephen Boyd <swboyd@...omium.org> wrote:
>
> Quoting Douglas Anderson (2023-08-24 08:30:30)
> > diff --git a/arch/arm64/include/asm/irq.h b/arch/arm64/include/asm/irq.h
> > index fac08e18bcd5..50ce8b697ff3 100644
> > --- a/arch/arm64/include/asm/irq.h
> > +++ b/arch/arm64/include/asm/irq.h
> > @@ -6,6 +6,9 @@
> >
> >  #include <asm-generic/irq.h>
> >
> > +void arch_trigger_cpumask_backtrace(const cpumask_t *mask, int exclude_cpu);
>
> Some nits, but otherwise
>
> Reviewed-by: Stephen Boyd <swboyd@...omium.org>
>
> > +#define arch_trigger_cpumask_backtrace arch_trigger_cpumask_backtrace
> > +
> >  struct pt_regs;
> >
> >  int set_handle_irq(void (*handle_irq)(struct pt_regs *));
> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > index a5848f1ef817..c8896cbc5327 100644
> > --- a/arch/arm64/kernel/smp.c
> > +++ b/arch/arm64/kernel/smp.c
> > @@ -72,12 +73,18 @@ enum ipi_msg_type {
> >         IPI_CPU_CRASH_STOP,
> >         IPI_TIMER,
> >         IPI_IRQ_WORK,
> > -       NR_IPI
> > +       NR_IPI,
> > +       /*
> > +        * Any enum >= NR_IPI and < MAX_IPI is special and not tracable
> > +        * with trace_ipi_*
> > +        */
> > +       IPI_CPU_BACKTRACE = NR_IPI,
> > +       MAX_IPI
> >  };
> >
> >  static int ipi_irq_base __read_mostly;
> >  static int nr_ipi __read_mostly = NR_IPI;
> > -static struct irq_desc *ipi_desc[NR_IPI] __read_mostly;
> > +static struct irq_desc *ipi_desc[MAX_IPI] __read_mostly;
>
> Side note: it would be nice to mark ipi_desc as __ro_after_init. Same
> for nr_ipi and ipi_irq_base.

I'd rather not change it in this patch since it's a pre-existing and
separate issue, but I can add a patch to the end of the series for
that if I end up spinning it. Otherwise I can send a follow-up patch
for it.


> >  static void ipi_setup(int cpu);
> >
> > @@ -845,6 +852,22 @@ static void __noreturn ipi_cpu_crash_stop(unsigned int cpu, struct pt_regs *regs
> >  #endif
> >  }
> >
> > +static void arm64_backtrace_ipi(cpumask_t *mask)
> > +{
> > +       __ipi_send_mask(ipi_desc[IPI_CPU_BACKTRACE], mask);
> > +}
> > +
> > +void arch_trigger_cpumask_backtrace(const cpumask_t *mask, int exclude_cpu)
>
> Can this be 'bool exclude_self' instead of int? That matches all other
> implementations from what I can tell.

Nope. See the part of the commit message that says:

This patch depends on commit 36759e343ff9 ("nmi_backtrace: allow
excluding an arbitrary CPU") since that commit changed the prototype
of arch_trigger_cpumask_backtrace(), which this patch implements.

> > +{
> > +       /*
> > +        * NOTE: though nmi_trigger_cpumask_backtrace has "nmi_" in the name,
>
> USe nmi_trigger_cpumask_backtrace() to indicate function.

I won't plan on doing an immediate spin for this and for now I'll wait
for additional feedback. If a maintainer is getting ready to land
this, I'm happy to post a new version with this fix or also happy if a
maintainer wants to add the "()" while applying.

-Doug