Re: [PATCH v4 2/2] samples: Add hung_task detector mutex blocking sample

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 26 Feb 2025 09:50:32 +0800
Lance Yang <ioworker0@gmail.com> wrote:

> On Tue, Feb 25, 2025 at 3:02 PM Masami Hiramatsu (Google)
> <mhiramat@kernel.org> wrote:
> >
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.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@kernel.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?

Yeah, I also have a question. This does not check the blocker,
but just providing the testing interface with mutex. So there is
no direct connection with DETECT_HUNG_TASK_BLOCKER. Thus I didn't
add the dependency.

Thank you,

> 
> 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");
> >


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>