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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 30 Jul 2025 17:36:04 +0800
Lance Yang <lance.yang@linux.dev> wrote:

> 
> 
> On 2025/7/30 16:51, Masami Hiramatsu (Google) wrote:
> > On Wed, 30 Jul 2025 16:59:22 +0900
> > Sergey Senozhatsky <senozhatsky@chromium.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.
> 
> The key issue is that we would lose the crucial T1 -> T2 relationship
> information from the log.

I just skip printing T2's stack dump, but still show "T1 is blocked by T2"
so the relationship is still clear.

Thank you,

> 
> While all three tasks would still be dumped, we would no longer be able
> to see the explicit dependency chain. It seems like the blocker tracking
> itself would be broken in this case.
> 
> Thanks,
> Lance
> 
> 
> 


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