linux-kernel - Re: [PATCH 1/2] hung_task: Show the blocker task if the task is hung on mutex

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Message-ID: <141d77e3-19e0-4f1b-9049-13dca0d5f339@redhat.com>
Date: Wed, 19 Feb 2025 20:40:02 -0500
From: Waiman Long <llong@...hat.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
 Steven Rostedt <rostedt@...dmis.org>
Cc: Waiman Long <llong@...hat.com>, 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>, Joel Granados <joel.granados@...nel.org>,
 Anna Schumaker <anna.schumaker@...cle.com>, Lance Yang
 <ioworker0@...il.com>, Kent Overstreet <kent.overstreet@...ux.dev>,
 Yongliang Gao <leonylgao@...cent.com>, Tomasz Figa <tfiga@...omium.org>,
 Sergey Senozhatsky <senozhatsky@...omium.org>, linux-kernel@...r.kernel.org,
 Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] hung_task: Show the blocker task if the task is hung
 on mutex

On 2/19/25 6:09 PM, Masami Hiramatsu (Google) wrote:
> On Wed, 19 Feb 2025 15:24:35 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
>
>> On Wed, 19 Feb 2025 15:18:57 -0500
>> Waiman Long <llong@...hat.com> wrote:
>>
>>> It is tricky to access the mutex_waiter structure which is allocated
>>> from stack. So another way to work around this issue is to add a new
>>> blocked_on_mutex field in task_struct to directly point to relevant
>>> mutex. Yes, that increase the size of task_struct by 8 bytes, but it is
>>> a pretty large structure anyway. Using READ_ONCE/WRITE_ONCE() to access
>> And it's been on my TODO list for some time to try to make that structure
>> smaller again :-/
>>
>>> this field, we don't need to take lock, though taking the wait_lock may
>>> still be needed to examine other information inside the mutex.
>> But perhaps if we add a new config option for this feature, we could just
>> add the lock that a task is blocked on before it goes to sleep and
>> reference that instead. That would be easier than trying to play games
>> getting the lock owner from the blocked_on field.
> So something like this?
>
> unsigned int	block_flags;
> union {
> 	struct mutex	*mutex;
> 	struct rwsem	+rwsem;
> 	struct rtmutex	*rtmutex;
> } blocked_on;
>
> enum {
> 	BLOCKED_ON_MUTEX;
> 	BLOCKED_ON_RWSEM;
> 	BLOCKED_ON_RTMUTEX;
> 	BLOCKED_ON_IO;
> } block_reason;

You should add one enum, e.g. BLOCKED_NONE, to represent the normal 
state of not being blocked.

Cheers,
Longman

> For the safety, we may anyway lock the task anyway, but that is the
> same as stacktrace.
>
> Thank you,
>
>> -- Steve
>