linux-kernel - Re: [PATCH 0/2] hung_task: Dump the blocking task stacktrace

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Message-Id: <20250220231857.6e377e5f208b60a7ba303cea@kernel.org>
Date: Thu, 20 Feb 2025 23:18:57 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Waiman Long <llong@...hat.com>
Cc: Lance Yang <ioworker0@...il.com>, 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>, Kent Overstreet
 <kent.overstreet@...ux.dev>, Yongliang Gao <leonylgao@...cent.com>, Steven
 Rostedt <rostedt@...dmis.org>, 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 0/2] hung_task: Dump the blocking task stacktrace

On Wed, 19 Feb 2025 15:20:39 -0500
Waiman Long <llong@...hat.com> wrote:

> 
> On 2/19/25 10:02 AM, Lance Yang wrote:
> > On Wed, Feb 19, 2025 at 9:33 PM Lance Yang <ioworker0@...il.com> wrote:
> >> CC linux-mm
> >>
> >> On Wed, Feb 19, 2025 at 9:00 PM Masami Hiramatsu (Google)
> >> <mhiramat@...nel.org> wrote:
> >>> Hi,
> >>>
> >>> The hung_task detector is very useful for detecting the lockup.
> >>> However, since it only dumps the blocked (uninterruptible sleep)
> >>> processes, it is not enough to identify the root cause of that
> >>> lockup.
> >>>
> >>> For example, if a process holds a mutex and sleep an event in
> >>> interruptible state long time, the other processes will wait on
> >>> the mutex in uninterruptible state. In this case, the waiter
> >>> processes are dumped, but the blocker process is not shown
> >>> because it is sleep in interruptible state.
> > Cool! I just ran into something similar today, but with rwsem. In that
> > case, the blocked process was locked up, and we could not identify
> > the root cause either ;(
> 
> Once this patch series is settled down, we can extend rwsem to provide 
> similar feature.

While discussing about rwsem with Sergey, he pointed that we can not
identify a single blocker on rwsem, because several readers can block
several writers. In this case, we need to dump all of them but we
don't have such info.

So anyway, I would like to start from mutex, which is the simplest one.
For the other locks, we will discuss later. (or start with limited
support, like showing only rwsem::owner)

Thanks,

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