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

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Date:   Wed, 3 Aug 2022 17:36:54 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     akpm@...ux-foundation.org, pmladek@...e.com,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, netdev@...r.kernel.org,
        x86@...nel.org, kernel-dev@...lia.com, kernel@...ccoli.net,
        halves@...onical.com, fabiomirmar@...il.com,
        alejandro.j.jimenez@...cle.com, andriy.shevchenko@...ux.intel.com,
        arnd@...db.de, bp@...en8.de, corbet@....net,
        d.hatayama@...fujitsu.com, dave.hansen@...ux.intel.com,
        dyoung@...hat.com, feng.tang@...el.com, gregkh@...uxfoundation.org,
        mikelley@...rosoft.com, hidehiro.kawai.ez@...achi.com,
        jgross@...e.com, john.ogness@...utronix.de, keescook@...omium.org,
        luto@...nel.org, mhiramat@...nel.org, mingo@...hat.com,
        paulmck@...nel.org, peterz@...radead.org, rostedt@...dmis.org,
        senozhatsky@...omium.org, stern@...land.harvard.edu,
        tglx@...utronix.de, vgoyal@...hat.com, vkuznets@...hat.com,
        will@...nel.org, Sergei Shtylyov <sergei.shtylyov@...il.com>
Subject: Re: [PATCH v2 08/13] tracing: Improve panic/die notifiers

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@...e.com>
> Cc: Sergei Shtylyov <sergei.shtylyov@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.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
>