linux-kernel - Re: [PATCH 3/5] kernel/watchdog: adapt the watchdog

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Date:   Fri, 17 Sep 2021 23:41:31 +0800
From:   Pingfan Liu <piliu@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Pingfan Liu <kernelfans@...il.com>, linux-kernel@...r.kernel.org,
        Sumit Garg <sumit.garg@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Julien Thierry <jthierry@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wang Qing <wangqing@...o.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Santosh Sivaraj <santosh@...six.org>
Subject: Re: [PATCH 3/5] kernel/watchdog: adapt the watchdog_hld interface
 for async model

On Thu, Sep 16, 2021 at 10:36:10AM +0200, Petr Mladek wrote:
> On Thu 2021-09-16 10:29:05, Petr Mladek wrote:
> > On Wed 2021-09-15 11:51:01, Pingfan Liu wrote:
> > > When lockup_detector_init()->watchdog_nmi_probe(), PMU may be not ready
> > > yet. E.g. on arm64, PMU is not ready until
> > > device_initcall(armv8_pmu_driver_init).  And it is deeply integrated
> > > with the driver model and cpuhp. Hence it is hard to push this
> > > initialization before smp_init().
> > > 
> > > But it is easy to take an opposite approach by enabling watchdog_hld to
> > > get the capability of PMU async.
> > 
> > This is another cryptic description. I have probably got it after
> > looking at the 5th patch (was not Cc :-(
> > 
> > > The async model is achieved by introducing an extra parameter notifier
> > > of watchdog_nmi_probe().
> > 
> > I would say that the code is horrible and looks too complex.
> > 
> > What about simply calling watchdog_nmi_probe() and
> > lockup_detector_setup() once again when watchdog_nmi_probe()
> > failed in lockup_detector_init()?
> > 
> > Or do not call lockup_detector_init() at all in
> > kernel_init_freeable() when PMU is not ready yet.
> 
> BTW: It is an overkill to create your own kthread just to run some
> code just once. And you implemeted it a wrong way. The kthread

I had thought about queue_work_on() in watchdog_nmi_enable(). But since
this work will block the worker kthread for this cpu. So finally,
another worker kthread should be created for other work.

But now, I think queue_work_on() may be more neat.

> must wait in a loop until someone else stop it and read
> the exit code.
> 
Is this behavior mandotory? Since this kthread can decide the exit
condition by itself.

And a quick through static int kthread(void *_create), I am not aware of
any problem with it.

> The easiest solution is to queue a work into system_wq for this.
> 
> I was not Cc for the 5th patch, so I write it here.
> 
Sorry for the inconvenience and Cc you now in case that you have further
comments. I will cc you in the next version.

Appreciate for all of your suggestions and comments

Thanks,

	Pingfan