linux-kernel - Re: [PATCH v9 7/7] arm64: kgdb: Roundup cpus using the debug IPI

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Date:   Mon, 7 Aug 2023 16:24:44 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Douglas Anderson <dianders@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sumit Garg <sumit.garg@...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>,
        Wei Li <liwei391@...wei.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 7/7] arm64: kgdb: Roundup cpus using the debug IPI

On Mon, Aug 07, 2023 at 11:28:52AM +0100, Mark Rutland wrote:
> On Thu, Jun 01, 2023 at 02:31:51PM -0700, Douglas Anderson wrote:
> > From: Sumit Garg <sumit.garg@...aro.org>
> >
> > Let's use the debug IPI for rounding up CPUs in kgdb. When the debug
> > IPI is backed by an NMI (or pseudo NMI) then this will let us debug
> > even hard locked CPUs. When the debug IPI isn't backed by an NMI then
> > this won't really have any huge benefit but it will still work.
> >
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > ---
> >
> > Changes in v9:
> > - Remove fallback for when debug IPI isn't available.
> > - Renamed "NMI IPI" to "debug IPI" since it might not be backed by NMI.
> >
> >  arch/arm64/kernel/ipi_debug.c |  5 +++++
> >  arch/arm64/kernel/kgdb.c      | 14 ++++++++++++++
> >  2 files changed, 19 insertions(+)
>
> This looks fine to me, but I'd feel a bit happier if we had separate SGIs for
> the backtrace and the KGDB callback as they're logically unrelated.

I've no objections to seperate SGIs (if one can be found) but I'm curious
what benefit emerges from giving them seperate IPIs.

Both interfaces are already designed to share and NMI-like IPI nicely
(and IIUC they must share one on x86), neither is performance
critical[1] and the content of /proc/interrupts for the IPI is seldom
going to be of much interest.

As mentioned it is perfectly OK to separate them if there is space in
the SGI allocations. However if any two functions are good candidates to
share a scarce resource such as an SGI then it is these!


Daniel.


[1] In both cases their results are only required at human-scale
    and the work of the both handlers is hugely more expensive than
    either up front quick-check.