linux-kernel - Re: [PATCH v3 1/2] nmi_backtrace: Allow excluding an arbitrary CPU

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Date:   Fri, 4 Aug 2023 17:02:14 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>, Arnd Bergmann <arnd@...db.de>,
        Borislav Petkov <bp@...en8.de>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Gaosheng Cui <cuigaosheng1@...wei.com>,
        "Gautham R. Shenoy" <gautham.shenoy@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Guo Ren <guoren@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Jianmin Lv <lvjianmin@...ngson.cn>,
        Jinyang He <hejinyang@...ngson.cn>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Pingfan Liu <kernelfans@...il.com>,
        Qing Zhang <zhangqing@...ngson.cn>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Russell King <linux@...linux.org.uk>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Rix <trix@...hat.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Valentin Schneider <vschneid@...hat.com>,
        WANG Xuerui <kernel@...0n.name>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        loongarch@...ts.linux.dev, sparclinux@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v3 1/2] nmi_backtrace: Allow excluding an arbitrary CPU

On Fri 04-08-23 06:56:51, Doug Anderson wrote:
> Hi,
> 
> On Fri, Aug 4, 2023 at 12:50 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Thu 03-08-23 16:07:57, Douglas Anderson wrote:
> > > The APIs that allow backtracing across CPUs have always had a way to
> > > exclude the current CPU. This convenience means callers didn't need to
> > > find a place to allocate a CPU mask just to handle the common case.
> > >
> > > Let's extend the API to take a CPU ID to exclude instead of just a
> > > boolean. This isn't any more complex for the API to handle and allows
> > > the hardlockup detector to exclude a different CPU (the one it already
> > > did a trace for) without needing to find space for a CPU mask.
> > >
> > > Arguably, this new API also encourages safer behavior. Specifically if
> > > the caller wants to avoid tracing the current CPU (maybe because they
> > > already traced the current CPU) this makes it more obvious to the
> > > caller that they need to make sure that the current CPU ID can't
> > > change.
> >
> > Yes, this looks like the best way forward.
> >
> > It would have been slightly safer to modify arch_trigger_cpumask_backtrace
> > by switching arguments so that some leftovers are captured easier.
> 
> I'm not sure I understand. Oh, you're saying make the prototype of
> arch_trigger_cpumask_backtrace() incompatible so that if someone is
> directly calling it then it'll be a compile-time error? 

exactly. bool to int promotion would be too easy to miss while the
pointer to int would complain loudly.

> I guess the
> hope is that nobody is calling that directly and they're calling
> through the trigger_...() functions.

Hope is one thing, being preventive another.

> For now I'm going to leave this alone.

If you are going to send another version then please consider this. Not
a hard requirement but better.
 

-- 
Michal Hocko
SUSE Labs