linux-kernel - Re: [PATCH v8 07/10] kgdb: Expose default CPUs roundup fallback mechanism

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Message-ID: <CAD=FV=WLAMv1rHiWWAoP8aTCpjWyzuHRuZtYAQRP3BwxFY0YNQ@mail.gmail.com>
Date:   Thu, 1 Jun 2023 14:47:48 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>, ito-yuichi@...itsu.com,
        kgdb-bugreport@...ts.sourceforge.net, Chen-Yu Tsai <wens@...e.org>,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        linux-arm-kernel@...ts.infradead.org,
        Stephen Boyd <swboyd@...omium.org>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-perf-users@...r.kernel.org,
        Jason Wessel <jason.wessel@...driver.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 07/10] kgdb: Expose default CPUs roundup fallback mechanism

Hi,

On Mon, May 15, 2023 at 4:21 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Fri, May 12, 2023 at 6:49 AM Daniel Thompson
> <daniel.thompson@...aro.org> wrote:
> >
> > On Wed, Apr 19, 2023 at 03:56:01PM -0700, Douglas Anderson wrote:
> > > From: Sumit Garg <sumit.garg@...aro.org>
> > >
> > > Add a new API kgdb_smp_call_nmi_hook() to expose default CPUs roundup
> > > mechanism to a particular archichecture as a runtime fallback if it
> > > detects to not support NMI roundup.
> > >
> > > Currently such an architecture example is arm64 supporting pseudo NMIs
> > > feature which is only available on platforms which have support for GICv3
> > > or later version.
> > >
> > > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > > Tested-by: Chen-Yu Tsai <wens@...e.org>
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > > ---
> > >
> > > (no changes since v1)
> > >
> > >  include/linux/kgdb.h      | 12 ++++++++++++
> > >  kernel/debug/debug_core.c |  8 +++++++-
> > >  2 files changed, 19 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> > > index 258cdde8d356..87713bd390f3 100644
> > > --- a/include/linux/kgdb.h
> > > +++ b/include/linux/kgdb.h
> > > @@ -199,6 +199,18 @@ kgdb_arch_handle_qxfer_pkt(char *remcom_in_buffer,
> > >
> > >  extern void kgdb_call_nmi_hook(void *ignored);
> > >
> > > +/**
> > > + *   kgdb_smp_call_nmi_hook - Provide default fallback mechanism to
> > > + *                            round-up CPUs
> > > + *
> > > + *   If you're using the default implementation of kgdb_roundup_cpus()
> > > + *   this function will be called.  And if an arch detects at runtime to
> > > + *   not support NMI based roundup then it can fallback to default
> > > + *   mechanism using this API.
> > > + */
> > > +
> > > +extern void kgdb_smp_call_nmi_hook(void);
> >
> > Concept looks sensible but this is a terrible name for aa command to
> > round up the CPUs using smp_call... functions. Whilst it is true it that
> > kgdb_roundup_cpus() does use kgdb_call_nmi_hook() internally that
> > doesn't mean we should name functions after it. They should be named
> > after what they are do, not how they do it.
> >
> > Something more like kgdb_roundup_cpus_with_smp_call() would be a much
> > better name.
>
> Sounds good. I'm happy to spin with this rename, though I was kinda
> hoping to drop ${SUBJECT} patch if folks were OK with patch #10 in
> this series [1]. I personally think that's the right way to go but
> it's unclear to me if arm64 maintainers will think it's a hack
> (despite the fact that arm32 implements the "nmi" functions with
> regular IPIs).
>
> For now, maybe I'll think positive thoughts and hope that folks will
> have the time to review the series soon. If another few weeks go by
> then I'll send a v9 with just your comments addressed. If nothing
> else, maybe you can land the kgdb parts in a tree targeting v6.5 and
> then when arm64 folks have the bandwidth then it will be easier to get
> them landed.
>
> [1] https://lore.kernel.org/r/20230419155341.v8.10.Ic3659997d6243139d0522fc3afcdfd88d7a5f030@changeid

Breadcrumbs: I've dropped this patch and several others in v9 [1] by
embracing the idea of using a normal IPI as a fallback.

[1] https://lore.kernel.org/r/20230601213440.2488667-1-dianders@chromium.org/