Re: [PATCH v2 08/13] tracing: Improve panic/die notifiers

Baoquan He
2022-08-03T09:37:08Z

On 07/19/22 at 04:53pm, Guilherme G. Piccoli wrote:
> Currently the tracing dump_on_oops feature is implemented
> through separate notifiers, one for die/oops and the other
> for panic - given they have the same functionality, let's
> unify them.
>
> Also improve the function comment and change the priority of
> the notifier to make it execute earlier, avoiding showing useless
> trace data (like the callback names for the other notifiers);
> finally, we also removed an unnecessary header inclusion.
>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Sergei Shtylyov <sergei.shtylyov@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
>
> ---
>
> V2:
> - Different approach; instead of using IDs to distinguish die and
> panic events, rely on address comparison like other notifiers do
> and as per Petr's suggestion;
>
> - Removed ACK from Steven since the code changed.
>
> kernel/trace/trace.c | 55 ++++++++++++++++++++++----------------------
> 1 file changed, 27 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index b8dd54627075..2a436b645c70 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -19,7 +19,6 @@
> #include <linux/kallsyms.h>
> #include <linux/security.h>
> #include <linux/seq_file.h>
> -#include <linux/notifier.h>
> #include <linux/irqflags.h>
> #include <linux/debugfs.h>
> #include <linux/tracefs.h>
> @@ -9777,40 +9776,40 @@ static __init int tracer_init_tracefs(void)
>
> fs_initcall(tracer_init_tracefs);
>
> -static int trace_panic_handler(struct notifier_block *this,
> - unsigned long event, void *unused)
> -{
> - if (ftrace_dump_on_oops)
> - ftrace_dump(ftrace_dump_on_oops);
> - return NOTIFY_OK;
> -}
> +static int trace_die_panic_handler(struct notifier_block *self,
> + unsigned long ev, void *unused);
>
> static struct notifier_block trace_panic_notifier = {
> - .notifier_call = trace_panic_handler,
> - .next = NULL,
> - .priority = 150 /* priority: INT_MAX >= x >= 0 */
> + .notifier_call = trace_die_panic_handler,
> + .priority = INT_MAX - 1,
> };
>
> -static int trace_die_handler(struct notifier_block *self,
> - unsigned long val,
> - void *data)
> -{
> - switch (val) {
> - case DIE_OOPS:
> - if (ftrace_dump_on_oops)
> - ftrace_dump(ftrace_dump_on_oops);
> - break;
> - default:
> - break;
> - }
> - return NOTIFY_OK;
> -}
> -
> static struct notifier_block trace_die_notifier = {
> - .notifier_call = trace_die_handler,
> - .priority = 200
> + .notifier_call = trace_die_panic_handler,
> + .priority = INT_MAX - 1,
> };
>
> +/*
> + * The idea is to execute the following die/panic callback early, in order
> + * to avoid showing irrelevant information in the trace (like other panic
> + * notifier functions); we are the 2nd to run, after hung_task/rcu_stall
> + * warnings get disabled (to prevent potential log flooding).
> + */
> +static int trace_die_panic_handler(struct notifier_block *self,
> + unsigned long ev, void *unused)
> +{
> + if (!ftrace_dump_on_oops)
> + goto out;
> +
> + if (self == &trace_die_notifier && ev != DIE_OOPS)
> + goto out;

Although the switch-case code of original trace_die_handler() is werid,
this unification is not much more comfortable. Just personal feeling
from code style, not strong opinion. Leave it to trace reviewers.

> +
> + ftrace_dump(ftrace_dump_on_oops);
> +
> +out:
> + return NOTIFY_DONE;
> +}
> +
> /*
> * printk is set to max of 1024, we really don't need it that big.
> * Nothing should be printing 1000 characters anyway.
> --
> 2.37.1
>