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Message-ID: <ZORY51mF4alI41G1@FVFF77S0Q05N>
Date: Tue, 22 Aug 2023 07:42:47 +0100
From: Mark Rutland <mark.rutland@....com>
To: Doug Anderson <dianders@...omium.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Sumit Garg <sumit.garg@...aro.org>,
Daniel Thompson <daniel.thompson@...aro.org>,
Marc Zyngier <maz@...nel.org>,
linux-perf-users@...r.kernel.org, ito-yuichi@...itsu.com,
Chen-Yu Tsai <wens@...e.org>, Ard Biesheuvel <ardb@...nel.org>,
Stephen Boyd <swboyd@...omium.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-arm-kernel@...ts.infradead.org,
kgdb-bugreport@...ts.sourceforge.net,
Masayoshi Mizuma <msys.mizuma@...il.com>,
"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
Lecopzer Chen <lecopzer.chen@...iatek.com>,
Andrey Konovalov <andreyknvl@...il.com>,
Masahiro Yamada <masahiroy@...nel.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 3/7] arm64: Add framework for a debug IPI
On Mon, Aug 21, 2023 at 03:16:56PM -0700, Doug Anderson wrote:
> Hi,
>
> On Mon, Aug 7, 2023 at 3:12 AM Mark Rutland <mark.rutland@....com> wrote:
> >
> > On Thu, Jun 01, 2023 at 02:31:47PM -0700, Douglas Anderson wrote:
> > > From: Sumit Garg <sumit.garg@...aro.org>
> > >
> > > Introduce a framework for an IPI that will be used for debug
> > > purposes. The primary use case of this IPI will be to generate stack
> > > crawls on other CPUs, but it will also be used to round up CPUs for
> > > kgdb.
> > >
> > > When possible, we try to allocate this debug IPI as an NMI (or a
> > > pseudo NMI). If that fails (due to CONFIG, an incompatible interrupt
> > > controller, a quirk, missing the "irqchip.gicv3_pseudo_nmi=1" kernel
> > > parameter, etc) we fall back to a normal IPI.
> > >
> > > NOTE: hooking this up for CPU backtrace / kgdb will happen in a future
> > > patch, this just adds the framework.
> > >
> > > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> >
> > I think that we shouldn't add a framework in a separate file for this:
> >
> > * This is very similar to our existing IPI management in smp.c, so it feels
> > like duplication, or at least another thing we'd like to keep in-sync.
> >
> > * We're going to want an NMI backtrace regardless of KGDB
> >
> > * We're going to want the IPI_CPU_STOP and IPI_CRASH_CPU_STOP IPIs to be NMIs
> > too.
> >
> > I reckon it'd be better to extend the existing IPI logic in smp.c to allow IPIs
> > to be requested as NMIs, e.g.
> >
> > ----
> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > index edd63894d61e8..48e6aa62c473e 100644
> > --- a/arch/arm64/kernel/smp.c
> > +++ b/arch/arm64/kernel/smp.c
> > @@ -33,6 +33,7 @@
> > #include <linux/kernel_stat.h>
> > #include <linux/kexec.h>
> > #include <linux/kvm_host.h>
> > +#include <linux/nmi.h>
> >
> > #include <asm/alternative.h>
> > #include <asm/atomic.h>
> > @@ -926,6 +927,21 @@ static void smp_cross_call(const struct cpumask *target, unsigned int ipinr)
> > __ipi_send_mask(ipi_desc[ipinr], target);
> > }
> >
> > +static bool ipi_should_be_nmi(enum ipi_msg_type ipi)
> > +{
> > + if (!system_uses_irq_prio_masking())
> > + return false;
> > +
> > + switch (ipi) {
> > + /*
> > + * TODO: select NMI IPIs here
> > + */
> > + return true;
> > + default:
> > + return false;
> > + }
> > +}
> > +
> > static void ipi_setup(int cpu)
> > {
> > int i;
> > @@ -933,8 +949,14 @@ static void ipi_setup(int cpu)
> > if (WARN_ON_ONCE(!ipi_irq_base))
> > return;
> >
> > - for (i = 0; i < nr_ipi; i++)
> > - enable_percpu_irq(ipi_irq_base + i, 0);
> > + for (i = 0; i < nr_ipi; i++) {
> > + if (ipi_should_be_nmi(i)) {
> > + prepare_percpu_nmi(ipi_irq_base + i);
> > + enable_percpu_nmi(ipi_irq_base + i, 0);
> > + } else {
> > + enable_percpu_irq(ipi_irq_base + i, 0);
> > + }
> > + }
> > }
> >
> > #ifdef CONFIG_HOTPLUG_CPU
> > @@ -945,8 +967,14 @@ static void ipi_teardown(int cpu)
> > if (WARN_ON_ONCE(!ipi_irq_base))
> > return;
> >
> > - for (i = 0; i < nr_ipi; i++)
> > - disable_percpu_irq(ipi_irq_base + i);
> > + for (i = 0; i < nr_ipi; i++) {
> > + if (ipi_should_be_nmi(i)) {
> > + disable_percpu_nmi(ipi_irq_base + i);
> > + teardown_percpu_nmi(ipi_irq_base + i);
> > + } else {
> > + disable_percpu_irq(ipi_irq_base + i);
> > + }
> > + }
> > }
> > #endif
> >
> > @@ -958,11 +986,19 @@ void __init set_smp_ipi_range(int ipi_base, int n)
> > nr_ipi = min(n, NR_IPI);
> >
> > for (i = 0; i < nr_ipi; i++) {
> > - int err;
> > -
> > - err = request_percpu_irq(ipi_base + i, ipi_handler,
> > - "IPI", &cpu_number);
> > - WARN_ON(err);
> > + int err = -EINVAL;
> > +
> > + if (ipi_should_be_nmi(i)) {
> > + err = request_percpu_nmi(ipi_base + i, ipi_handler,
> > + "IPI", &cpu_number);
> > + WARN(err, "Could not request IPI %d as NMI, err=%d\n",
> > + i, err);
> > + } else {
> > + err = request_percpu_irq(ipi_base + i, ipi_handler,
> > + "IPI", &cpu_number);
> > + WARN(err, "Could not request IPI %d as IRQ, err=%d\n",
> > + i, err);
> > + }
> >
> > ipi_desc[i] = irq_to_desc(ipi_base + i);
> > irq_set_status_flags(ipi_base + i, IRQ_HIDDEN);
> > ----
> >
> > ... and then if we need an IPI for KGDB, we can add that to the existing list
> > of IPIs, and have it requested/enabled/disabled as usual.
>
> Sounds good. I'm starting to work on v10 incorporating your feedback.
> A few quick questions:
>
> 1. If I mostly take your patch above verbatim, do you have any
> suggested tags for Author/Signed-off-by? I'd tend to set you as the
> author but I can't do that because you didn't provide a
> Signed-off-by...
Sorry about that. For the above:
Signed-off-by: Mark Rutland <mark.rutland@....com>
If squashed into another patch, then feel free to use:
Co-developed-by: Mark Rutland <mark.rutland@....com>
> 2. Would you prefer this patch on its own, or would you rather it be
> squashed with the first user ("backtrace")? On its own, I think I have
> to get rid of the "switch" statement in ipi_should_be_nmi() and just
> return false;
I reckon it makes sense to squash it with the first user.
Thanks,
Mark.
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