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Message-ID: <CAK1f24k7hRD5SZD92tv+DgSakhA8vRjtLibGTymd9AsHizTBAg@mail.gmail.com>
Date: Tue, 18 Mar 2025 19:35:32 +0800
From: Lance Yang <ioworker0@...il.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: akpm@...ux-foundation.org, will@...nel.org, peterz@...radead.org,
mingo@...hat.com, longman@...hat.com, 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 Tue, Mar 18, 2025 at 5:42 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> 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.
Thanks for taking time to review!
>
> >
> > 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.
Hmm... Looking back, adding it was probably overkill. So let's drop it in
the next version ;)
>
> > +
> > + /*
> > + * 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;
Ah, I didn't realize we could do it that way before. That's nice to know!
>
> > +
> > + 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.
Good catch! I completely missed it ;(
>
> > +
> > /*
> > * 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.
Yes, you're right. Will remove that as you suggested.
Thanks,
Lance
>
> > +
> > 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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