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Message-Id: <20250318184158.e6c1b2d28e2fee311bcd2c30@kernel.org>
Date: Tue, 18 Mar 2025 18:41:58 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, will@...nel.org, peterz@...radead.org,
mingo@...hat.com, longman@...hat.com, mhiramat@...nel.org,
anna.schumaker@...cle.com, boqun.feng@...il.com, joel.granados@...nel.org,
kent.overstreet@...ux.dev, leonylgao@...cent.com,
linux-kernel@...r.kernel.org, rostedt@...dmis.org,
senozhatsky@...omium.org, tfiga@...omium.org, amaindex@...look.com, Mingzhe
Yang <mingzhe.yang@...com>
Subject: Re: [PATCH RESEND v2 1/3] hung_task: replace blocker_mutex with
encoded blocker
On Fri, 14 Mar 2025 22:42:58 +0800
Lance Yang <ioworker0@...il.com> wrote:
> This patch replaces 'struct mutex *blocker_mutex' with 'unsigned long
> blocker', as only one blocker is active at a time.
>
> The blocker filed can store both the lock addrees and the lock type, with
> LSB used to encode the type as Masami suggested, making it easier to extend
> the feature to cover other types of locks.
>
> Also, once the lock type is determined, we can directly extract the address
> and cast it to a lock pointer ;)
Hi Lance,
Thanks for update. I added some comments.
>
> Suggested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Signed-off-by: Mingzhe Yang <mingzhe.yang@...com>
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
> include/linux/hung_task.h | 94 +++++++++++++++++++++++++++++++++++++++
> include/linux/sched.h | 2 +-
> kernel/hung_task.c | 15 ++++---
> kernel/locking/mutex.c | 8 +++-
> 4 files changed, 111 insertions(+), 8 deletions(-)
> create mode 100644 include/linux/hung_task.h
>
> diff --git a/include/linux/hung_task.h b/include/linux/hung_task.h
> new file mode 100644
> index 000000000000..64ced33b0d1f
> --- /dev/null
> +++ b/include/linux/hung_task.h
> @@ -0,0 +1,94 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Detect Hung Task: detecting tasks stuck in D state
> + *
> + * Copyright (C) 2025 Tongcheng Travel (www.ly.com)
> + * Author: Lance Yang <mingzhe.yang@...com>
> + */
> +#ifndef __LINUX_HUNG_TASK_H
> +#define __LINUX_HUNG_TASK_H
> +
> +#include <linux/bug.h>
> +#include <linux/sched.h>
> +#include <linux/compiler.h>
> +
> +/*
> + * @blocker: Combines lock address and blocking type.
> + *
> + * Since lock pointers are at least 4-byte aligned(32-bit) or 8-byte
> + * aligned(64-bit). This leaves the 2 least bits (LSBs) of the pointer
> + * always zero. So we can use these bits to encode the specific blocking
> + * type.
> + *
> + * Type encoding:
> + * 00 - Blocked on mutex (BLOCKER_TYPE_MUTEX)
> + * 01 - Blocked on semaphore (BLOCKER_TYPE_SEM)
> + * 10 - Blocked on rt-mutex (BLOCKER_TYPE_RTMUTEX)
> + * 11 - Blocked on rw-semaphore (BLOCKER_TYPE_RWSEM)
> + */
> +#define BLOCKER_TYPE_MUTEX 0x00UL
> +#define BLOCKER_TYPE_SEM 0x01UL
> +#define BLOCKER_TYPE_RTMUTEX 0x02UL
> +#define BLOCKER_TYPE_RWSEM 0x03UL
> +
> +#define BLOCKER_TYPE_MASK 0x03UL
> +
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +static inline void hung_task_set_blocker(void *lock, unsigned long type)
> +{
> + unsigned long lock_ptr = (unsigned long)lock;
> +
> + WARN_ON_ONCE(!lock_ptr);
> + WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK);
> + WARN_ON_ONCE(READ_ONCE(current->blocker));
The last one needs a comment why it is problem.
WARN_ON_ONCE(READ_ONCE(current->blocker),
"Task blocker is not cleared. Maybe forgot to clear it somewhere? Type: %d\n",
current->blocker);
But I don't think this isn't needed to be checked usually.
> +
> + /*
> + * If the lock pointer matches the BLOCKER_TYPE_MASK, return
> + * without writing anything.
> + */
> + if (lock_ptr & BLOCKER_TYPE_MASK)
> + return;
You can use WARN_ON_ONCE for 'if' condition.
if (WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK))
return;
> +
> + WRITE_ONCE(current->blocker, lock_ptr | type);
> +}
> +
> +static inline void hung_task_clear_blocker(void)
> +{
> + WARN_ON_ONCE(!READ_ONCE(current->blocker));
> +
> + WRITE_ONCE(current->blocker, 0UL);
> +}
> +
> +static inline bool hung_task_blocker_is_type(unsigned long blocker,
> + unsigned long type)
> +{
> + WARN_ON_ONCE(!blocker);
> +
> + return (blocker & BLOCKER_TYPE_MASK) == type;
> +}
> +
> +static inline void *hung_task_blocker_to_lock(unsigned long blocker)
> +{
> + WARN_ON_ONCE(!blocker);
> +
> + return (void *)(blocker & ~BLOCKER_TYPE_MASK);
> +}
> +#else
> +static inline void hung_task_set_blocker(void *lock, unsigned long type)
> +{
> +}
> +static inline void hung_task_clear_blocker(void)
> +{
> +}
> +static inline bool hung_task_blocker_is_type(unsigned long blocker,
> + unsigned long type)
> +{
> + return false;
> +}
> +static inline void *hung_task_blocker_to_lock(unsigned long blocker)
> +{
> + return NULL;
> +}
> +#endif
> +
> +#endif /* __LINUX_HUNG_TASK_H */
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 1419d94c8e87..f27060dac499 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1218,7 +1218,7 @@ struct task_struct {
> #endif
>
> #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> - struct mutex *blocker_mutex;
> + unsigned long blocker;
> #endif
>
> #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index dc898ec93463..46eb6717564d 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -25,6 +25,10 @@
>
> #include <trace/events/sched.h>
>
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#include <linux/hung_task.h>
> +#endif
We don't need this #ifdef, since it is already checked in the
header file.
> +
> /*
> * The number of tasks checked:
> */
> @@ -98,16 +102,17 @@ static struct notifier_block panic_block = {
> static void debug_show_blocker(struct task_struct *task)
> {
> struct task_struct *g, *t;
> - unsigned long owner;
> - struct mutex *lock;
> + unsigned long owner, blocker;
>
> RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "No rcu lock held");
>
> - lock = READ_ONCE(task->blocker_mutex);
> - if (!lock)
> + blocker = READ_ONCE(task->blocker);
> + if (!blocker || !hung_task_blocker_is_type(blocker, BLOCKER_TYPE_MUTEX))
> return;
>
> - owner = mutex_get_owner(lock);
> + owner = mutex_get_owner(
> + (struct mutex *)hung_task_blocker_to_lock(blocker));
> +
> if (unlikely(!owner)) {
> pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not found.\n",
> task->comm, task->pid);
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 6a543c204a14..642d6398e0dd 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -42,6 +42,10 @@
> # define MUTEX_WARN_ON(cond)
> #endif
>
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#include <linux/hung_task.h>
> +#endif
Ditto.
> +
> void
> __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
> {
> @@ -189,7 +193,7 @@ __mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
> struct list_head *list)
> {
> #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> - WRITE_ONCE(current->blocker_mutex, lock);
> + hung_task_set_blocker(lock, BLOCKER_TYPE_MUTEX);
> #endif
> debug_mutex_add_waiter(lock, waiter, current);
>
> @@ -207,7 +211,7 @@ __mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter)
>
> debug_mutex_remove_waiter(lock, waiter, current);
> #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> - WRITE_ONCE(current->blocker_mutex, NULL);
> + hung_task_clear_blocker();
> #endif
> }
>
> --
> 2.45.2
>
Thank you,
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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