linux-kernel - Re: [PATCH v2] hung_task: Dump blocker task if it is not hung

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Message-ID: <reyd4bppb5tfon7gtqaelwknvptdeyhrsh5ijbcj77ezini3yq@ivkgxmiqzk3x>
Date: Thu, 31 Jul 2025 11:53:51 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.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 v2] hung_task: Dump blocker task if it is not hung

On (25/07/31 07:11), Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Dump the lock blocker task if it is not hung because if the blocker
> task is also hung, it should be dumped by the detector. This will
> de-duplicate the same stackdumps if the blocker task is also blocked
> by another task (and hung).
> 
> Suggested-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Acked-by: Lance Yang <lance.yang@...ux.dev>

Tested-by: Sergey Senozhatsky <senozhatsky@...omium.org>


Wrote a simple 3 tasks circular lock test (same that I had in real
life).  The output looks good:

[   90.985431] [    T140] INFO: task T1:525 blocked for more than 30 seconds.
[   90.990981] [    T140]       Not tainted 6.16.0-next-20250730-00003-g5865c79d6085-dirty #261
[   90.996912] [    T140] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[   91.001617] [    T140] task:T1              state:D stack:0     pid:525   tgid:525   ppid:2      task_flags:0x208040 flags:0x00004000
[   91.007564] [    T140] Call Trace:
[   91.008767] [    T140]  <TASK>
[   91.009872] [    T140]  __schedule+0x137f/0x20c0
[   91.011606] [    T140]  schedule+0xdc/0x280
[   91.013115] [    T140]  schedule_preempt_disabled+0x10/0x20
[   91.015200] [    T140]  __mutex_lock+0x721/0x1590
[   91.016817] [    T140]  ? __mutex_lock+0x500/0x1590
[   91.018074] [    T140]  mutex_lock+0x81/0x90
[   91.019169] [    T140]  t1+0x62/0x70
[   91.020061] [    T140]  kthread+0x583/0x6e0
[   91.021140] [    T140]  ? drop_caches_sysctl_handler+0x130/0x130
[   91.022729] [    T140]  ? kthread_blkcg+0xa0/0xa0
[   91.023921] [    T140]  ret_from_fork+0xc8/0x160
[   91.025091] [    T140]  ? kthread_blkcg+0xa0/0xa0
[   91.026309] [    T140]  ret_from_fork_asm+0x11/0x20
[   91.027577] [    T140]  </TASK>
[   91.028326] [    T140] INFO: task T1:525 is blocked on a mutex likely owned by task T2:526.
[   91.030404] [    T140] INFO: task T2:526 blocked for more than 30 seconds.
[   91.031978] [    T140]       Not tainted 6.16.0-next-20250730-00003-g5865c79d6085-dirty #261
[   91.034069] [    T140] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[   91.036215] [    T140] task:T2              state:D stack:0     pid:526   tgid:526   ppid:2      task_flags:0x208040 flags:0x00004000
[   91.039233] [    T140] Call Trace:
[   91.040084] [    T140]  <TASK>
[   91.040810] [    T140]  __schedule+0x137f/0x20c0
[   91.041954] [    T140]  schedule+0xdc/0x280
[   91.042984] [    T140]  schedule_preempt_disabled+0x10/0x20
[   91.044329] [    T140]  __mutex_lock+0x721/0x1590
[   91.045464] [    T140]  ? __mutex_lock+0x500/0x1590
[   91.046624] [    T140]  mutex_lock+0x81/0x90
[   91.047632] [    T140]  t2+0x69/0x70
[   91.048473] [    T140]  kthread+0x583/0x6e0
[   91.049453] [    T140]  ? drop_pagecache_sb+0x200/0x200
[   91.050686] [    T140]  ? kthread_blkcg+0xa0/0xa0
[   91.051808] [    T140]  ret_from_fork+0xc8/0x160
[   91.052886] [    T140]  ? kthread_blkcg+0xa0/0xa0
[   91.054036] [    T140]  ret_from_fork_asm+0x11/0x20
[   91.055205] [    T140]  </TASK>
[   91.056011] [    T140] INFO: task T2:526 is blocked on a mutex likely owned by task T3:527.
[   91.058014] [    T140] INFO: task T3:527 blocked for more than 30 seconds.
[   91.059682] [    T140]       Not tainted 6.16.0-next-20250730-00003-g5865c79d6085-dirty #261
[   91.061708] [    T140] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[   91.063825] [    T140] task:T3              state:D stack:0     pid:527   tgid:527   ppid:2      task_flags:0x208040 flags:0x00004000
[   91.066668] [    T140] Call Trace:
[   91.067462] [    T140]  <TASK>
[   91.068121] [    T140]  __schedule+0x137f/0x20c0
[   91.069244] [    T140]  schedule+0xdc/0x280
[   91.070264] [    T140]  schedule_preempt_disabled+0x10/0x20
[   91.071723] [    T140]  __mutex_lock+0x721/0x1590
[   91.072848] [    T140]  ? __mutex_lock+0x500/0x1590
[   91.074032] [    T140]  mutex_lock+0x81/0x90
[   91.075054] [    T140]  t3+0x23/0x30
[   91.075932] [    T140]  kthread+0x583/0x6e0
[   91.076953] [    T140]  ? t2+0x70/0x70
[   91.077865] [    T140]  ? kthread_blkcg+0xa0/0xa0
[   91.078974] [    T140]  ret_from_fork+0xc8/0x160
[   91.080109] [    T140]  ? kthread_blkcg+0xa0/0xa0
[   91.081250] [    T140]  ret_from_fork_asm+0x11/0x20
[   91.082530] [    T140]  </TASK>
[   91.083248] [    T140] INFO: task T3:527 is blocked on a mutex likely owned by task T1:525.