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: <20250220080908.fc1494f0f7c611b48fbe0f8b@kernel.org>
Date: Thu, 20 Feb 2025 08:09:08 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Waiman Long <llong@...hat.com>, "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 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;

For the safety, we may anyway lock the task anyway, but that is the
same as stacktrace.

Thank you,

> 
> -- Steve


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