Coccinelle archive on
 help / color / Atom feed
From: Vlastimil Babka <>
To: "Guilherme G. Piccoli" <>,,
Subject: Re: [Cocci] [PATCH V2] kernel/hung_task.c: Introduce sysctl to print all traces when a hung task is detected
Date: Tue, 24 Mar 2020 09:27:37 +0100
Message-ID: <> (raw)
In-Reply-To: <>

On 3/23/20 10:46 PM, Guilherme G. Piccoli wrote:
> Commit 401c636a0eeb ("kernel/hung_task.c: show all hung tasks before panic")
> introduced a change in that we started to show all CPUs backtraces when a
> hung task is detected _and_ the sysctl/kernel parameter "hung_task_panic"
> is set. The idea is good, because usually when observing deadlocks (that
> may lead to hung tasks), the culprit is another task holding a lock and
> not necessarily the task detected as hung.
> The problem with this approach is that dumping backtraces is a slightly
> expensive task, specially printing that on console (and specially in many
> CPU machines, as servers commonly found nowadays). So, users that plan to
> collect a kdump to investigate the hung tasks and narrow down the deadlock
> definitely don't need the CPUs backtrace on dmesg/console, which will delay
> the panic and pollute the log (crash tool would easily grab all CPUs traces
> with 'bt -a' command).
> Also, there's the reciprocal scenario: some users may be interested in
> seeing the CPUs backtraces but not have the system panic when a hung task
> is detected. The current approach hence is almost as embedding a policy in
> the kernel, by forcing the CPUs backtraces' dump (only) on hung_task_panic.
> This patch decouples the panic event on hung task from the CPUs backtraces
> dump, by creating (and documenting) a new sysctl/kernel parameter called
> "hung_task_all_cpu_backtrace", analog to the approach taken on soft/hard
> lockups, that have both a panic and an "all_cpu_backtrace" sysctl to allow
> individual control. The new mechanism for dumping the CPUs backtraces on
> hung task detection respects "hung_task_warnings" by not dumping the
> traces in case there's no warnings left.
> Cc: Tetsuo Handa <>
> Signed-off-by: Guilherme G. Piccoli <>
> ---
> V2: Followed suggestions from Kees and Tetsuo (and other grammar
> improvements). Also, followed Tetsuo suggestion to itereate kernel
> testing community - but I don't really know a ML for that, so I've
> CCed Coccinelle community and kernel-api ML.
> Also, Tetsuo suggested that this option could be default to 1 - I'm
> open to it, but given it is only available if hung_task panic is set
> as of now and the goal of this patch is give users more flexibility,
> I vote to keep default as 0. I can respin a V3 in case more people
> want to see it enabled by default. Thanks in advance for the review!
> Cheers,
> Guilherme
>  .../admin-guide/kernel-parameters.txt         |  6 ++++
>  Documentation/admin-guide/sysctl/kernel.rst   | 15 ++++++++++
>  include/linux/sched/sysctl.h                  |  7 +++++
>  kernel/hung_task.c                            | 30 +++++++++++++++++--
>  kernel/sysctl.c                               | 11 +++++++
>  5 files changed, 67 insertions(+), 2 deletions(-)
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c07815d230bc..7a14caac6c94 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1453,6 +1453,12 @@
>  			x86-64 are 2M (when the CPU supports "pse") and 1G
>  			(when the CPU supports the "pdpe1gb" cpuinfo flag).
> +	hung_task_all_cpu_backtrace=
> +			[KNL] Should kernel generate backtraces on all cpus
> +			when a hung task is detected. Defaults to 0 and can
> +			be controlled by hung_task_all_cpu_backtrace sysctl.
> +			Format: <integer>
> +

Before adding a new thing as both kernel parameter and sysctl, could we perhaps
not add the kernel parameter, in favor of the generic sysctl parameter solution?
[1] There were no objections and some support from Kees, so I will try to send a
new version ASAP that will work properly with all "static" sysctls - we don't
need to be blocked by a full solution for dynamically registered sysctls yet, I



Cocci mailing list

  parent reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-23 21:46 Guilherme G. Piccoli
2020-03-23 21:51 ` Kees Cook
2020-03-23 23:39 ` Randy Dunlap
2020-03-24  8:27 ` Vlastimil Babka [this message]
2020-03-24 12:45   ` Guilherme G. Piccoli
2020-03-24 18:20     ` Kees Cook
2020-03-25  9:33       ` Vlastimil Babka

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Coccinelle archive on

Archives are clonable:
	git clone --mirror cocci/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 cocci cocci/ \
	public-inbox-index cocci

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone