linux-kernel - Re: [PATCH v4 1/2] hung_task: Show the blocker task if the task is hung on mutex

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Message-ID: <npvzweqkpwqgcjzmb4h7ymp57m5kjrziwyl7pqeb2pgqeysbma@op25ggpv7ce5>
Date: Wed, 30 Jul 2025 19:16:31 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Lance Yang <lance.yang@...ux.dev>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, 
	Sergey Senozhatsky <senozhatsky@...omium.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>, 
	Waiman Long <longman@...hat.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>, 
	Steven Rostedt <rostedt@...dmis.org>, Tomasz Figa <tfiga@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] hung_task: Show the blocker task if the task is
 hung on mutex

On (25/07/30 17:36), Lance Yang wrote:
> On 2025/7/30 16:51, Masami Hiramatsu (Google) wrote:
> > On Wed, 30 Jul 2025 16:59:22 +0900
> > Sergey Senozhatsky <senozhatsky@...omium.org> wrote:
> > 
> > > One thing that gives me a bit of "inconvenience" is that in certain
> > > cases this significantly increases the amount of stack traces to go
> > > through.  A distilled real life example:
> > > - task T1 acquires lock L1, attempts to acquire L2
> > > - task T2 acquires lock L2, attempts to acquire L3
> > > - task T3 acquires lock L3, attempts to acquire L1
> > > 
> > > So we'd now see:
> > > - a backtrace of T1, followed by a backtrace of T2 (owner of L2)
> > > - a backtrace of T2, followed by a backtrace of T3 (owner of L3)
> > > - a backtrace of T3, followed by a backtrace of T1 (owner of L1)
> > > 
> > > Notice how each task is backtraced twice.  I wonder if it's worth it
> > > to de-dup the backtraces.  E.g. in
> > > 
> > > 	task cat:115 is blocked on a mutex likely owned by task cat:114
> > > 
> > > if we know that cat:114 is also blocked on a lock, then we probably
> > > can just say "is blocked on a mutex likely owned by task cat:114" and
> > > continue iterating through tasks.  That "cat:114" will be backtraced
> > > individually later, as it's also blocked on a lock, owned by another
> > > task.
> > > 
> > > Does this make any sense?
> > 
> > Hrm, OK. So what about dump the blocker task only if that task is
> > NOT blocked? (because if the task is blocked, it should be dumped
> > afterwards (or already))
> 
> Hmm... I'm concerned about a potential side effect of that logic.
> 
> Consider a simple, non-circular blocking chain like T1 -> T2 -> T3.
> 
> In this scenario, T1, T2, and T3 would all be dumped as hung tasks.
> However, with the proposed rule (dump only if NOT blocked), when the
> detector processes T1, it would see that its blocker (T2) is also
> blocked and would therefore skip printing any blocker information about
> T2.

That's not what I proposed.  The suggestions here is to print only
"is blocked likely owned by task cat:114" and do not append the
backtrace of that cat:114, because it will be printed separately
(since it's a blocked task).  But we should do so only if blocker
is also blocked.  So the relation between T1 and T2 will still be
exposed.