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    SubjectRe: [PATCH 2/3] panic: Add option to dump all CPUs backtraces in panic_print
    On Tue, Nov 09, 2021 at 05:28:47PM -0300, Guilherme G. Piccoli wrote:
    > Currently the "panic_print" parameter/sysctl allows some interesting debug
    > information to be printed during a panic event. This is useful for example
    > in cases the user cannot kdump due to resource limits, or if the user
    > collects panic logs in a serial output (or pstore) and prefers a fast
    > reboot instead of a kdump.
    >
    > Happens that currently there's no way to see all CPUs backtraces in
    > a panic using "panic_print" on architectures that support that. We do
    > have "oops_all_cpu_backtrace" sysctl, but although partially overlapping
    > in the functionality, they are orthogonal in nature: "panic_print" is
    > a panic tuning (and we have panics without oopses, like direct calls to
    > panic() or maybe other paths that don't go through oops_enter()
    > function), and the original purpose of "oops_all_cpu_backtrace" is to
    > provide more information on oopses for cases in which the users desire
    > to continue running the kernel even after an oops, i.e., used in
    > non-panic scenarios.
    >
    > So, we hereby introduce an additional bit for "panic_print" to allow
    > dumping the CPUs backtraces during a panic event.

    This looks to be helpful for debugging panic.

    Reviewed-by: Feng Tang <feng.tang@intel.com>

    Thanks,
    Feng


    > Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
    > ---
    > Documentation/admin-guide/kernel-parameters.txt | 1 +
    > Documentation/admin-guide/sysctl/kernel.rst | 1 +
    > kernel/panic.c | 4 ++++
    > 3 files changed, 6 insertions(+)
    >
    > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
    > index 0905d2cdb2d5..569d035c4332 100644
    > --- a/Documentation/admin-guide/kernel-parameters.txt
    > +++ b/Documentation/admin-guide/kernel-parameters.txt
    > @@ -3690,6 +3690,7 @@
    > bit 3: print locks info if CONFIG_LOCKDEP is on
    > bit 4: print ftrace buffer
    > bit 5: print all printk messages in buffer
    > + bit 6: print all CPUs backtrace (if available in the arch)
    >
    > panic_on_taint= Bitmask for conditionally calling panic() in add_taint()
    > Format: <hex>[,nousertaint]
    > diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
    > index 70b7df9b081a..1666c1a9dbba 100644
    > --- a/Documentation/admin-guide/sysctl/kernel.rst
    > +++ b/Documentation/admin-guide/sysctl/kernel.rst
    > @@ -796,6 +796,7 @@ bit 2 print timer info
    > bit 3 print locks info if ``CONFIG_LOCKDEP`` is on
    > bit 4 print ftrace buffer
    > bit 5: print all printk messages in buffer
    > +bit 6: print all CPUs backtrace (if available in the arch)
    > ===== ============================================
    >
    > So for example to print tasks and memory info on panic, user can::
    > diff --git a/kernel/panic.c b/kernel/panic.c
    > index cefd7d82366f..5da71fa4e5f1 100644
    > --- a/kernel/panic.c
    > +++ b/kernel/panic.c
    > @@ -65,6 +65,7 @@ EXPORT_SYMBOL_GPL(panic_timeout);
    > #define PANIC_PRINT_LOCK_INFO 0x00000008
    > #define PANIC_PRINT_FTRACE_INFO 0x00000010
    > #define PANIC_PRINT_ALL_PRINTK_MSG 0x00000020
    > +#define PANIC_PRINT_ALL_CPU_BT 0x00000040
    > unsigned long panic_print;
    >
    > ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
    > @@ -151,6 +152,9 @@ static void panic_print_sys_info(void)
    > if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
    > console_flush_on_panic(CONSOLE_REPLAY_ALL);
    >
    > + if (panic_print & PANIC_PRINT_ALL_CPU_BT)
    > + trigger_all_cpu_backtrace();
    > +
    > if (panic_print & PANIC_PRINT_TASK_INFO)
    > show_state();
    >
    > --
    > 2.33.1

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