linux-kernel - Re: [PATCH v9 0/7] arm64: Add debug IPI for backtraces / kgdb; try to use NMI for it

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Message-ID: <ZNDKVP2m-iiZCz3v@FVFF77S0Q05N.cambridge.arm.com>
Date:   Mon, 7 Aug 2023 11:41:24 +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>,
        Ben Dooks <ben-linux@...ff.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Gautham R. Shenoy" <gautham.shenoy@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Wei Li <liwei391@...wei.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 0/7] arm64: Add debug IPI for backtraces / kgdb; try
 to use NMI for it

Hi Doug,

Apologies for the delay.

On Mon, Jul 24, 2023 at 08:55:44AM -0700, Doug Anderson wrote:
> On Thu, Jun 1, 2023 at 2:37 PM Douglas Anderson <dianders@...omium.org> wrote:
> I'm looking for some ideas on what to do to move this patch series
> forward. Thanks to Daniel, the kgdb patch is now in Linus's tree which
> hopefully makes this simpler to land. I guess there is still the
> irqchip dependency that will need to be sorted out, though...
> 
> Even if folks aren't in agreement about whether this is ready to be
> enabled in production, I don't think anything here is super
> objectionable or controversial, is it? Can we land it? If you feel
> like it needs extra review, would it help if I tried to drum up some
> extra people to provide review feedback?

Ignoring the soundness issues I mentioned before (which I'm slowly chipping
away at, and you're likely lucky enough to avoid in practice)...

Having looked over the series, I think the GICv3 bit isn't quite right, but is
easy enough to fix. I've commented on the patch with what I think we should
have there.

The only major thing otherwise from my PoV is the structure of the debug IPI
framework. I'm not keen on that being a separate body of code and I think it
should live in smp.c along with the other IPIs. I'd also strongly prefer if we
could have separate IPI_CPU_BACKTRACE and IPI_CPU_KGDB IPIs, and I think we can
do that either by unifying IPI_CPU_STOP && IPI_CPU_CRASH_STOP or by reclaiming
IPI_WAKEUP by reusing a different IPI for the parking protocol (e.g.
IPI_RESCHEDULE).

I think it'd be nice if the series could enable NMIs for backtrace and the
CPU_{,CRASH_}STOP cases, with KGDB being the bonus atop. That way it'd be
clearly beneficial for anyone trying to debug lockups even if they're not a
KGDB user.

> Also: in case it's interesting to anyone, I've been doing benchmarks
> on sc7180-trogdor devices in preparation for enabling this. On that
> platform, I did manage to see about 4% reduction in a set of hackbench
> numbers when fully enabling pseudo-NMI. However, when I instead ran
> Speedometer 2.1 I saw no difference. See:
> 
> https://issuetracker.google.com/issues/197061987

Thanks for the pointer!

I know that there are a couple of things that we could do to slightly improve
local_irq_*() when using pNMIs, though I suspect that the bulk of the cost
there will come from the necessary synchronization.

Thanks,
Mark.