linux-kernel - Re: [PATCH v5 06/18] watchdog/hardlockup: Add comments to touch_nmi

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Date:   Tue, 23 May 2023 11:58:13 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Stephane Eranian <eranian@...gle.com>, mpe@...erman.id.au,
        Tzung-Bi Shih <tzungbi@...omium.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        linuxppc-dev@...ts.ozlabs.org, Sumit Garg <sumit.garg@...aro.org>,
        npiggin@...il.com, davem@...emloft.net,
        Marc Zyngier <maz@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>, sparclinux@...r.kernel.org,
        christophe.leroy@...roup.eu,
        Catalin Marinas <catalin.marinas@....com>,
        ravi.v.shankar@...el.com, Randy Dunlap <rdunlap@...radead.org>,
        Pingfan Liu <kernelfans@...il.com>,
        Guenter Roeck <groeck@...omium.org>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Ian Rogers <irogers@...gle.com>, ito-yuichi@...itsu.com,
        ricardo.neri@...el.com, linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, Will Deacon <will@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>, linux-kernel@...r.kernel.org,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v5 06/18] watchdog/hardlockup: Add comments to
 touch_nmi_watchdog()

On Fri 2023-05-19 10:18:30, Douglas Anderson wrote:
> In preparation for the buddy hardlockup detector, add comments to
> touch_nmi_watchdog() to make it obvious that it touches the configured
> hardlockup detector regardless of whether it's backed by an NMI. Also
> note that arch_touch_nmi_watchdog() may not be architecture-specific.
> 
> Ideally, we'd like to rename these functions but that is a fairly
> disruptive change touching a lot of drivers. After discussion [1] the
> plan is to defer this until a good time.
> 
> [1] https://lore.kernel.org/r/ZFy0TX1tfhlH8gxj@alley
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> 
> Changes in v5:
> - No longer rename touch_nmi_watchdog(), just add comments.
> 
> Changes in v4:
> - ("Rename touch_nmi_watchdog() to ...") new for v4.
> 
>  include/linux/nmi.h | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/nmi.h b/include/linux/nmi.h
> index 454fe99c4874..fafab128f37e 100644
> --- a/include/linux/nmi.h
> +++ b/include/linux/nmi.h
> @@ -125,15 +125,30 @@ void watchdog_nmi_disable(unsigned int cpu);
>  void lockup_detector_reconfigure(void);
>  
>  /**
> - * touch_nmi_watchdog - restart NMI watchdog timeout.
> + * touch_nmi_watchdog - manually pet the hardlockup watchdog.
>   *
> - * If the architecture supports the NMI watchdog, touch_nmi_watchdog()
> - * may be used to reset the timeout - for code which intentionally
> - * disables interrupts for a long time. This call is stateless.
> + * If we support detecting hardlockups, touch_nmi_watchdog() may be
> + * used to pet the watchdog (reset the timeout) - for code which

Nit: I personally prefer "reset the timeout" over "pet the watchdog".
     "pet" is just another ambiguous name as "touch" ;-)

> + * intentionally disables interrupts for a long time. This call is stateless.
> + *
> + * Though this function has "nmi" in the name, the hardlockup watchdog might
> + * not be backed by NMIs. This function will likely be renamed to
> + * touch_hardlockup_watchdog() in the future.
>   */
>  static inline void touch_nmi_watchdog(void)
>  {
> +	/*
> +	 * Pass on to the hardlockup detector selected via CONFIG_. Note that
> +	 * the hardlockup detector may not be arch-specific nor using NMIs
> +	 * and the arch_touch_nmi_watchdog() function will likely be renamed
> +	 * in the future.
> +	 */
>  	arch_touch_nmi_watchdog();
> +
> +	/*
> +	 * Touching the hardlock detector implcitily pets the
> +	 * softlockup detector too
> +	 */

s/implcitily/implicitly/

That said, I would remove this comment completely. It describes what
is clear from the code.

A more useful information would be why it is done. But it is probably
clear as well. CPU could not schedule when interrupts are disabled.

>  	touch_softlockup_watchdog();
>  }

With the removed comment above touch_softlockup_watchdog():

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr