Message-ID: <CAD=FV=U=ox4ApMbDL7v=ivNF6x=UyG=dd4MU_Dt0rppNCEwCpw@mail.gmail.com>
Date:   Fri, 16 Jun 2023 09:48:06 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        sparclinux@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 4/6] watchdog/hardlockup: Make HAVE_NMI_WATCHDOG sparc64-specific

Hi,

On Fri, Jun 16, 2023 at 8:07 AM Petr Mladek <pmladek@...e.com> wrote:
>
> There are several hardlockup detector implementations and several Kconfig
> values which allow selection and build of the preferred one.
>
> CONFIG_HARDLOCKUP_DETECTOR was introduced by the commit 23637d477c1f53acb
> ("lockup_detector: Introduce CONFIG_HARDLOCKUP_DETECTOR") in v2.6.36.
> It was a preparation step for introducing the new generic perf hardlockup
> detector.
>
> The existing arch-specific variants did not support the to-be-created
> generic build configurations, sysctl interface, etc. This distinction
> was made explicit by the commit 4a7863cc2eb5f98 ("x86, nmi_watchdog:
> Remove ARCH_HAS_NMI_WATCHDOG and rely on CONFIG_HARDLOCKUP_DETECTOR")
> in v2.6.38.
>
> CONFIG_HAVE_NMI_WATCHDOG was introduced by the commit d314d74c695f967e105
> ("nmi watchdog: do not use cpp symbol in Kconfig") in v3.4-rc1. It replaced
> the above mentioned ARCH_HAS_NMI_WATCHDOG. At that time, it was still used
> by three architectures, namely blackfin, mn10300, and sparc.
>
> The support for blackfin and mn10300 architectures has been completely
> dropped some time ago. And sparc is the only architecture with the historic
> NMI watchdog at the moment.
>
> And the old sparc implementation is really special. It is always built on
> sparc64. It used to be always enabled until the commit 7a5c8b57cec93196b
> ("sparc: implement watchdog_nmi_enable and watchdog_nmi_disable") added
> in v4.10-rc1.
>
> There are only few locations where the sparc64 NMI watchdog interacts
> with the generic hardlockup detectors code:
>
>   + implements arch_touch_nmi_watchdog() which is called from the generic
>     touch_nmi_watchdog()
>
>   + implements watchdog_hardlockup_enable()/disable() to support
>     /proc/sys/kernel/nmi_watchdog
>
>   + is always preferred over other generic watchdogs, see
>     CONFIG_HARDLOCKUP_DETECTOR
>
>   + includes asm/nmi.h into linux/nmi.h because some sparc-specific
>     functions are needed in sparc-specific code which includes
>     only linux/nmi.h.
>
> The situation became more complicated after the commit 05a4a95279311c3
> ("kernel/watchdog: split up config options") and commit 2104180a53698df5
> ("powerpc/64s: implement arch-specific hardlockup watchdog") in v4.13-rc1.
> They introduced HAVE_HARDLOCKUP_DETECTOR_ARCH. It was used for powerpc
> specific hardlockup detector. It was compatible with the perf one
> regarding the general boot, sysctl, and programming interfaces.
>
> HAVE_HARDLOCKUP_DETECTOR_ARCH was defined as a superset of
> HAVE_NMI_WATCHDOG. It made some sense because all arch-specific
> detectors had some common requirements, namely:
>
>   + implemented arch_touch_nmi_watchdog()
>   + included asm/nmi.h into linux/nmi.h
>   + defined the default value for /proc/sys/kernel/nmi_watchdog
>
> But it actually has made things pretty complicated when the generic
> buddy hardlockup detector was added. Before the generic perf detector
> was newer supported together with an arch-specific one. But the buddy
> detector could work on any SMP system. It means that an architecture
> could support both the arch-specific and buddy detector.
>
> As a result, there are few tricky dependencies. For example,
> CONFIG_HARDLOCKUP_DETECTOR depends on:
>
>   ((HAVE_HARDLOCKUP_DETECTOR_PERF || HAVE_HARDLOCKUP_DETECTOR_BUDDY) && !HAVE_NMI_WATCHDOG) || HAVE_HARDLOCKUP_DETECTOR_ARCH
>
> The problem is that the very special sparc implementation is defined as:
>
>   HAVE_NMI_WATCHDOG && !HAVE_HARDLOCKUP_DETECTOR_ARCH
>
> Another problem is that the meaning of HAVE_NMI_WATCHDOG is far from clear
> without reading understanding the history.
>
> Make the logic less tricky and more self-explanatory by making
> HAVE_NMI_WATCHDOG specific for the sparc64 implementation. And rename it to
> HAVE_HARDLOCKUP_DETECTOR_SPARC64.
>
> Note that HARDLOCKUP_DETECTOR_PREFER_BUDDY, HARDLOCKUP_DETECTOR_PERF,
> and HARDLOCKUP_DETECTOR_BUDDY may conflict only with
> HAVE_HARDLOCKUP_DETECTOR_ARCH. They depend on HARDLOCKUP_DETECTOR
> and it is not longer enabled when HAVE_NMI_WATCHDOG is set.
>
> Signed-off-by: Petr Mladek <pmladek@...e.com>
>
> watchdog/sparc64: Rename HAVE_NMI_WATCHDOG to HAVE_HARDLOCKUP_WATCHDOG_SPARC64
>
> The configuration variable HAVE_NMI_WATCHDOG has a generic name but
> it is selected only for SPARC64.
>
> It should _not_ be used in general because it is not integrated with
> the other hardlockup detectors. Namely, it does not support the hardlockup
> specific command line parameters and systcl interface. Instead, it is
> enabled/disabled together with the softlockup detector by the global
> "watchdog" sysctl.
>
> Rename it to HAVE_HARDLOCKUP_WATCHDOG_SPARC64 to make the special
> behavior more clear.
>
> Also the variable is set only on sparc64. Move the definition
> from arch/Kconfig to arch/sparc/Kconfig.debug.
>
> Signed-off-by: Petr Mladek <pmladek@...e.com>

I think you goofed up when squashing the patches. You've now got a
second patch subject after your first Signed-off-by and then a second
Signed-off-by... I assume everything after the first Signed-off-by
should be dropped?

Other than that:

Reviewed-by: Douglas Anderson <dianders@...omium.org>

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