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: <20250220075639.298616eb494248d390417977@kernel.org>
Date: Thu, 20 Feb 2025 07:56:39 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Waiman Long <llong@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, "Masami Hiramatsu (Google)"
 <mhiramat@...nel.org>, 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 Wed, 19 Feb 2025 17:44:11 -0500
Waiman Long <llong@...hat.com> wrote:

> 
> On 2/19/25 3:24 PM, Steven Rostedt 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 :-/

I agree to add the field, actually it was my first prototype :)

> >
> >> 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.

Do we need to take it just for accessing owner, which is in an atomic?

> > 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.
> 
> Yes, it could be a new config option. This will be a useful feature that 
> I believe most distros will turn it on. Or we may just include that in 
> the core code without any option.

Do we need another option? or just extend DETECT_HUNG_TASK?

Thanks,

> 
> BTW, this field can also be shared by other sleeping locks like rwsem 
> and rt_mutex as a task can only be blocked on one of them. We do need 
> another type field to identify the type of the blocked lock.
> 
> Cheers,
> Longman
> 
> 


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