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Message-ID: <CAK1f24nLDHLChnwG14rNa5RYik-3+Sqz74OAR1PsWvg3Zu_2+w@mail.gmail.com>
Date: Wed, 26 Feb 2025 09:50:32 +0800
From: Lance Yang <ioworker0@...il.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Will Deacon <will@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
Joel Granados <joel.granados@...nel.org>, Anna Schumaker <anna.schumaker@...cle.com>,
Kent Overstreet <kent.overstreet@...ux.dev>, Yongliang Gao <leonylgao@...cent.com>,
Steven Rostedt <rostedt@...dmis.org>, Tomasz Figa <tfiga@...omium.org>,
Sergey Senozhatsky <senozhatsky@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] samples: Add hung_task detector mutex blocking sample
On Tue, Feb 25, 2025 at 3:02 PM Masami Hiramatsu (Google)
<mhiramat@...nel.org> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
> Add a hung_task detector mutex blocking test sample code.
>
> This module will create a dummy file on the debugfs. That file will
> cause the read process to sleep for enough long time (256 seconds)
> while holding a mutex. As a result, the second process will wait on
> the mutex for a prolonged duration and be detected by the hung_task
> detector.
>
> Usage is;
>
> > cd /sys/kernel/debug/hung_task
> > cat mutex & cat mutex
>
> and wait for hung_task message.
>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> ---
> samples/Kconfig | 9 +++++
> samples/Makefile | 1 +
> samples/hung_task/Makefile | 2 +
> samples/hung_task/hung_task_mutex.c | 66 +++++++++++++++++++++++++++++++++++
> 4 files changed, 78 insertions(+)
> create mode 100644 samples/hung_task/Makefile
> create mode 100644 samples/hung_task/hung_task_mutex.c
>
> diff --git a/samples/Kconfig b/samples/Kconfig
> index 820e00b2ed68..09011be2391a 100644
> --- a/samples/Kconfig
> +++ b/samples/Kconfig
> @@ -300,6 +300,15 @@ config SAMPLE_CHECK_EXEC
> demonstrate how they should be used with execveat(2) +
> AT_EXECVE_CHECK.
>
> +config SAMPLE_HUNG_TASK
> + tristate "Hung task detector test code"
> + depends on DETECT_HUNG_TASK && DEBUG_FS
> + help
> + Build a module which provide a simple debugfs file. If user reads
> + the file, it will sleep long time (256 seconds) with holding a
> + mutex. Thus if there are 2 or more processes read this file, it
> + will be detected by the hung_task watchdog.
> +
> source "samples/rust/Kconfig"
I'm just wondering if we should also make the SAMPLE_HUNG_TASK
depend on !PREEMPT_RT for now?
Thanks,
Lance
>
> source "samples/damon/Kconfig"
> diff --git a/samples/Makefile b/samples/Makefile
> index f24cd0d72dd0..bf6e6fca5410 100644
> --- a/samples/Makefile
> +++ b/samples/Makefile
> @@ -42,3 +42,4 @@ obj-$(CONFIG_SAMPLE_FPROBE) += fprobe/
> obj-$(CONFIG_SAMPLES_RUST) += rust/
> obj-$(CONFIG_SAMPLE_DAMON_WSSE) += damon/
> obj-$(CONFIG_SAMPLE_DAMON_PRCL) += damon/
> +obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task/
> diff --git a/samples/hung_task/Makefile b/samples/hung_task/Makefile
> new file mode 100644
> index 000000000000..fe9dde799880
> --- /dev/null
> +++ b/samples/hung_task/Makefile
> @@ -0,0 +1,2 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_mutex.o
> \ No newline at end of file
> diff --git a/samples/hung_task/hung_task_mutex.c b/samples/hung_task/hung_task_mutex.c
> new file mode 100644
> index 000000000000..7a29f2246d22
> --- /dev/null
> +++ b/samples/hung_task/hung_task_mutex.c
> @@ -0,0 +1,66 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * hung_task_mutex.c - Sample code which causes hung task by mutex
> + *
> + * Usage: load this module and read `<debugfs>/hung_task/mutex`
> + * by 2 or more processes.
> + *
> + * This is for testing kernel hung_task error message.
> + * Note that this will make your system freeze and maybe
> + * cause panic. So do not use this except for the test.
> + */
> +
> +#include <linux/debugfs.h>
> +#include <linux/delay.h>
> +#include <linux/fs.h>
> +#include <linux/module.h>
> +#include <linux/mutex.h>
> +
> +#define HUNG_TASK_DIR "hung_task"
> +#define HUNG_TASK_FILE "mutex"
> +#define SLEEP_SECOND 256
> +
> +static const char dummy_string[] = "This is a dummy string.";
> +static DEFINE_MUTEX(dummy_mutex);
> +struct dentry *hung_task_dir;
> +
> +static ssize_t read_dummy(struct file *file, char __user *user_buf,
> + size_t count, loff_t *ppos)
> +{
> + /* If the second task waits on the lock, it is uninterruptible sleep. */
> + guard(mutex)(&dummy_mutex);
> +
> + /* When the first task sleep here, it is interruptible. */
> + msleep_interruptible(SLEEP_SECOND * 1000);
> +
> + return simple_read_from_buffer(user_buf, count, ppos,
> + dummy_string, sizeof(dummy_string));
> +}
> +
> +static const struct file_operations hung_task_fops = {
> + .read = read_dummy,
> +};
> +
> +static int __init hung_task_sample_init(void)
> +{
> + hung_task_dir = debugfs_create_dir(HUNG_TASK_DIR, NULL);
> + if (IS_ERR(hung_task_dir))
> + return PTR_ERR(hung_task_dir);
> +
> + debugfs_create_file(HUNG_TASK_FILE, 0400, hung_task_dir,
> + NULL, &hung_task_fops);
> +
> + return 0;
> +}
> +
> +static void __exit hung_task_sample_exit(void)
> +{
> + debugfs_remove_recursive(hung_task_dir);
> +}
> +
> +module_init(hung_task_sample_init);
> +module_exit(hung_task_sample_exit);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Masami Hiramatsu");
> +MODULE_DESCRIPTION("Simple sleep under mutex file for testing hung task");
>
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