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Date:   Wed, 22 Sep 2021 12:26:57 +0800
From:   Pingfan Liu <piliu@redhat.com>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Pingfan Liu <kernelfans@gmail.com>, linux-kernel@vger.kernel.org,
        Sumit Garg <sumit.garg@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Julien Thierry <jthierry@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wang Qing <wangqing@vivo.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Santosh Sivaraj <santosh@fossix.org>
Subject: Re: [PATCH 3/5] kernel/watchdog: adapt the watchdog_hld interface
 for async model
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On Mon, Sep 20, 2021 at 10:20:46AM +0200, Petr Mladek wrote:
> On Fri 2021-09-17 23:41:31, Pingfan Liu wrote:
[...]
> > 
> > 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.
> 
> This is not a problem. workqueues use a pool of workers that are
> already created and can be used when one worker gets blocked.
> 
Yes, you are right. The creation is dynamic and immune to blocking.

> > 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.
> 
> I am pretty sure. Unfortunately, I can't find it in the documentation.
> 
> My view is the following. Each process has a task_struct. The
> scheduler needs task_struct so that it can switch processes.
> The task_struct must still exist when the process exits.
> The scheduler puts the task into TASK_DEAD state.
> Another process has to read the exit code and destroy the
> task struct.
> 
Thanks for bringing up this, and I have an opportunity to think about it.

The core of the problem is put_task_struct(), and who releases the
last one.
It should be: finish_task_switch()->put_task_struct_rcu_user()->delayed_put_task_struct()->put_task_struct(),
if (unlikely(prev_state == TASK_DEAD)). It does not depend on another task.

> See, do_exit() in kernel/exit.c. It ends with do_dead_task().
> It is the point when the process goes into TASK_DEAD state.
> 
> For a good example, see lib/test_vmalloc.c. The kthread waits
> until anyone want him to stop:
> 
> static int test_func(void *private)
> {
> [...]
> 
> 	/*
> 	 * Wait for the kthread_stop() call.
> 	 */
> 	while (!kthread_should_stop())
> 		msleep(10);
> 
> 	return 0;
> }
> 
> The kthreads are started and stopped in:
> 
> static void do_concurrent_test(void)
> {
> [...]
> 	for (i = 0; i < nr_threads; i++) {
> [...]
> 		t->task = kthread_run(test_func, t, "vmalloc_test/%d", i);
> [...]
> 	/*
> 	 * Sleep quiet until all workers are done with 1 second
> 	 * interval. Since the test can take a lot of time we
> 	 * can run into a stack trace of the hung task. That is
> 	 * why we go with completion_timeout and HZ value.
> 	 */
> 	do {
> 		ret = wait_for_completion_timeout(&test_all_done_comp, HZ);
> 	} while (!ret);
> [...]
> 	for (i = 0; i < nr_threads; i++) {
> [...]
> 		if (!IS_ERR(t->task))
> 			kthread_stop(t->task);
> [...]
> }

They are good and elegant examples.
> 
> 
> You do not have to solve this if you use the system workqueue
> (system_wq).
> 
Yes, workqueue is a better choice.

Thanks for your great patience.


Regards,

	Pingfan