linux-kernel - Re: [PATCH] kfence: Avoid stalling work queue task without allocations

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Date:   Mon, 23 Nov 2020 11:36:59 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Marco Elver <elver@...gle.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kasan-dev <kasan-dev@...glegroups.com>, rcu@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] kfence: Avoid stalling work queue task without
 allocations

On Mon, 23 Nov 2020 11:28:12 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> I noticed:
> 
> 
> [  237.650900] enabling event benchmark_event
> 
> In both traces. Could you disable CONFIG_TRACEPOINT_BENCHMARK and see if
> the issue goes away. That event kicks off a thread that spins in a tight
> loop for some time and could possibly cause some issues.
> 
> It still shouldn't break things, we can narrow it down if it is the culprit.

And it probably is the issue because that thread will never sleep! It runs
a loop of:


static int benchmark_event_kthread(void *arg)
{
	/* sleep a bit to make sure the tracepoint gets activated */
	msleep(100);

	while (!kthread_should_stop()) {

		trace_do_benchmark();

		/*
		 * We don't go to sleep, but let others run as well.
		 * This is basically a "yield()" to let any task that
		 * wants to run, schedule in, but if the CPU is idle,
		 * we'll keep burning cycles.
		 *
		 * Note the tasks_rcu_qs() version of cond_resched() will
		 * notify synchronize_rcu_tasks() that this thread has
		 * passed a quiescent state for rcu_tasks. Otherwise
		 * this thread will never voluntarily schedule which would
		 * block synchronize_rcu_tasks() indefinitely.
		 */
		cond_resched_tasks_rcu_qs();
	}

	return 0;
}


Did something change, where that "cond_resched_tasks_rcu_qs()" doesn't let
things progress on ARM64?

I noticed that you have PREEMPT enabled so this will only be preempted when
its schedule time runs out and something else wants to run. How would that
affect other threads?

-- Steve