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: <db4ee5e9-56bb-408c-85e7-f93e2c3226dc@redhat.com>
Date: Wed, 19 Feb 2025 17:44:11 -0500
From: Waiman Long <llong@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>, 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 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 :-/
>
>> 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.

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.

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