From: "Guilherme G. Piccoli" gpiccoli@canonical.com

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 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.

Signed-off-by: Guilherme G. Piccoli gpiccoli@canonical.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Reviewed-by: Kees Cook keescook@chromium.org Cc: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Link: http://lkml.kernel.org/r/20200327223646.20779-1-gpiccoli@canonical.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Bixuan Cui cuibixuan@huawei.com --- Documentation/sysctl/kernel.txt | 14 ++++++++++++++ include/linux/sched/sysctl.h | 7 +++++++ kernel/hung_task.c | 20 ++++++++++++++++++-- kernel/sysctl.c | 11 +++++++++++ 4 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index f41ae5a3cc8d..7e33d039ea17 100644 --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -337,6 +337,20 @@ Default value is "/sbin/hotplug".

==============================================================

+hung_task_all_cpu_backtrace: + +If this option is set, the kernel will send an NMI to all CPUs to dump +their backtraces when a hung task is detected. This file shows up if +CONFIG_DETECT_HUNG_TASK and CONFIG_SMP are enabled. + +0: Won't show all CPUs backtraces when a hung task is detected. +This is the default behavior. + +1: Will non-maskably interrupt all CPUs and dump their backtraces when +a hung task is detected. + +============================================================== + hung_task_panic:

Controls the kernel's behavior when a hung task is detected. diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h index 4993c00824a3..cabdc1ed134a 100644 --- a/include/linux/sched/sysctl.h +++ b/include/linux/sched/sysctl.h @@ -7,6 +7,13 @@ struct ctl_table;

#ifdef CONFIG_DETECT_HUNG_TASK + +#ifdef CONFIG_SMP +extern unsigned int sysctl_hung_task_all_cpu_backtrace; +#else +#define sysctl_hung_task_all_cpu_backtrace 2 +#endif /* CONFIG_SMP */ + extern int sysctl_hung_task_check_count; extern unsigned int sysctl_hung_task_panic; extern unsigned long sysctl_hung_task_timeout_secs; diff --git a/kernel/hung_task.c b/kernel/hung_task.c index 4a9191617076..8a2842bfa4e1 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -51,9 +51,18 @@ int __read_mostly sysctl_hung_task_warnings = 10; static int __read_mostly did_panic; static bool hung_task_show_lock; static bool hung_task_call_panic; +static bool hung_task_show_all_bt;

static struct task_struct *watchdog_task;

+#ifdef CONFIG_SMP +/* + * Should we dump all CPUs backtraces in a hung task event? + * Defaults to 0, can be changed via sysctl. + */ +unsigned int __read_mostly sysctl_hung_task_all_cpu_backtrace; +#endif /* CONFIG_SMP */ + /* * Should we panic (and reboot, if panic_timeout= is set) when a * hung task is detected: @@ -135,6 +144,9 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout) " disables this message.\n"); sched_show_task(t); hung_task_show_lock = true; + + if (sysctl_hung_task_all_cpu_backtrace) + hung_task_show_all_bt = true; }

touch_nmi_watchdog(); @@ -199,10 +211,14 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout) rcu_read_unlock(); if (hung_task_show_lock) debug_show_all_locks(); - if (hung_task_call_panic) { + + if (hung_task_show_all_bt) { + hung_task_show_all_bt = false; trigger_all_cpu_backtrace(); - panic("hung_task: blocked tasks"); } + + if (hung_task_call_panic) + panic("hung_task: blocked tasks"); }

static long hung_timeout_jiffies(unsigned long last_checked, diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 1733b8c71b11..dbd8ef22a48b 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -1070,6 +1070,17 @@ static struct ctl_table kern_table[] = { }, #endif #ifdef CONFIG_DETECT_HUNG_TASK +#ifdef CONFIG_SMP + { + .procname = "hung_task_all_cpu_backtrace", + .data = &sysctl_hung_task_all_cpu_backtrace, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec_minmax, + .extra1 = &zero, + .extra2 = &one, + }, +#endif /* CONFIG_SMP */ { .procname = "hung_task_panic", .data = &sysctl_hung_task_panic,