linux-kernel - Re: [PATCH v2 09/13] notifier: Show function names on notifier routines if DEBUG

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Date:   Tue, 19 Jul 2022 13:48:43 -0700
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        akpm@...ux-foundation.org, bhe@...hat.com, pmladek@...e.com,
        kexec@...ts.infradead.org
Cc:     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,
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        senozhatsky@...omium.org, stern@...land.harvard.edu,
        tglx@...utronix.de, vgoyal@...hat.com, vkuznets@...hat.com,
        will@...nel.org, Cong Wang <xiyou.wangcong@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Xiaoming Ni <nixiaoming@...wei.com>
Subject: Re: [PATCH v2 09/13] notifier: Show function names on notifier
 routines if DEBUG_NOTIFIERS is set

On 7/19/2022 1:44 PM, Guilherme G. Piccoli wrote:
> On 19/07/2022 17:33, Arjan van de Ven wrote:
>> On 7/19/2022 12:53 PM, Guilherme G. Piccoli wrote:
>>> Currently we have a debug infrastructure in the notifiers file, but
>>> it's very simple/limited. Extend it by:
>>>
>>> (a) Showing all registered/unregistered notifiers' callback names;
>>
>>
>> I'm not yet convinced that this is the right direction.
>> The original intent for this "debug" feature was to be lightweight enough that it could run in production, since at the time, rootkits
>> liked to clobber/hijack notifiers and there were also some other signs of corruption at the time.
>>
>> By making something print (even at pr_info) for what are probably frequent non-error operations, you turn something that is light
>> into something that's a lot more heavy and generally that's not a great idea... it'll be a performance surprise.
>>
>>
> 
> Is registering/un-registering notifiers a hot path, or performance
> sensitive usually? For me, this patch proved to be very useful, and once
> enabled, shows relatively few entries in dmesg, these operations aren't
> so common thing it seems.
> 
> Also, if this Kconfig option was meant to run in production, maybe the
> first thing would be have some sysfs tuning or anything able to turn it
> on - I've worked with a variety of customers and the most terrifying
> thing in servers is to install a new kernel and reboot heh
> 
> My understanding is that this debug infrastructure would be useful for
> notifiers writers and people playing with the core notifiers
> code...tracing would be much more useful in the context of checking if
> some specific notifier got registered/executed in production environment
> I guess.

I would totally support an approach where instead of pr_info, there's a tracepoint
for these events (and that shouldnt' need to be conditional on a config option)

that's not what the patch does though.