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

[Lance Yang]

On 2025/7/30 18:16, Sergey Senozhatsky wrote:
> 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@xxxxxxxxxxxx> 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.

You're right, thanks for clarifying! I misunderstood the key detail :(

Thanks,
Lance