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

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Message-ID: <YupFeQ6AcfjUVpOW@MiWiFi-R3L-srv>
Date:   Wed, 3 Aug 2022 17:52:57 +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 08/03/22 at 05:36pm, Baoquan He wrote:
> 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.

Please ignore this comment.

I use b4 to grab this patchset and applied, and started to check patch
one by one. Then I realize it's all about cleanups which have got
consensus in earlier rounds. Hope it can be merged when other people's
concern is addressed, the whole series looks good to me, I have no
strong concern to them.

> 
> > +
> > +	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
> > 
>