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Date:   Tue, 19 Jul 2022 13:48:43 -0700
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Subject: Re: [PATCH v2 09/13] notifier: Show function names on notifier
 routines if DEBUG_NOTIFIERS is set
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To:     "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        akpm@linux-foundation.org, bhe@redhat.com, pmladek@suse.com,
        kexec@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
        netdev@vger.kernel.org, x86@kernel.org, kernel-dev@igalia.com,
        kernel@gpiccoli.net, halves@canonical.com, fabiomirmar@gmail.com,
        alejandro.j.jimenez@oracle.com, andriy.shevchenko@linux.intel.com,
        arnd@arndb.de, bp@alien8.de, corbet@lwn.net,
        d.hatayama@jp.fujitsu.com, dave.hansen@linux.intel.com,
        dyoung@redhat.com, feng.tang@intel.com, gregkh@linuxfoundation.org,
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        tglx@linutronix.de, vgoyal@redhat.com, vkuznets@redhat.com,
        will@kernel.org, Cong Wang <xiyou.wangcong@gmail.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Xiaoming Ni <nixiaoming@huawei.com>
References: <20220719195325.402745-1-gpiccoli@igalia.com>
 <20220719195325.402745-10-gpiccoli@igalia.com>
 <e292e128-d732-e770-67d7-b6ed947cec7b@linux.intel.com>
 <8e201d99-78a8-d68c-6d33-676a1ba5a6ee@igalia.com>
From:   Arjan van de Ven <arjan@linux.intel.com>
In-Reply-To: <8e201d99-78a8-d68c-6d33-676a1ba5a6ee@igalia.com>
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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.