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: <20250219152435.35077ac3@gandalf.local.home>
Date: Wed, 19 Feb 2025 15:24:35 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Waiman Long <llong@...hat.com>
Cc: "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 15:18:57 -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.

-- Steve