Unused trace event in erofs

[Hongbo Li]

On 2025/6/13 20:17, Steven Rostedt wrote:
> On Fri, 13 Jun 2025 14:08:32 +0800
> Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
> 
>> Hi Steven,
>>
>> On 2025/6/13 10:49, Steven Rostedt wrote:
>>> I have code that will trigger a warning if a trace event is defined but
>>> not used[1]. It gives a list of unused events. Here's what I have for
>>> erofs:
>>>
>>> warning: tracepoint 'erofs_destroy_inode' is unused.
>>
>> I'm fine to remove it, also I wonder if it's possible to disable
>> erofs tracepoints (rather than disable all tracepoints) in some
>> embedded use cases because erofs tracepoints might not be useful for
>> them and it can save memory (and .ko size) as you said below.
> 
> You can add #ifdef around them.
> 
Should we introduce a new CONFIG to disable them?

Thanks,
Hongbo

> Note, the "up to around 5K" means it can add up to that much depending on
> what you have configured. The TRACE_EVENT() macro (and more specifically
> the DECLARE_EVENT_CLASS() which TRACE_EVENT() has), is where all the bloat
> is. I generates unique code for each trace event that prints it, parses it,
> records it, the event fields, and has code specific for perf, ftrace and BPF.
> 
> The DEFINE_EVENT() which can be used to make several events that are
> similar use the same DECLARE_EVENT_CLASS() only takes up around 250 bytes.
> One reason I tell people to use DECLARE_EVENT_CLASS() when you have similar events.
> 
> There's also a DEFINE_EVENT_PRINT() that can use an existing
> DECLARE_EVENT_CLASS() but update the "printk" section. That adds some more
> code (the creation of the print function) but still much smaller than the
> DECLARE_EVENT_CLASS(). But this requires the tracepoint function (what the
> code calls) must have the same prototype.
> 
>>
>>>
>>> Each trace event can take up to around 5K in memory regardless if they
>>> are used or not. Soon there will be warnings when they are defined but
>>> not used. Please remove any unused trace event or at least hide it
>>> under an #ifdef if they are used within configs. I'm planning on adding
>>> these warning in the next merge window.
>>
>> If you don't have some interest to submit a removal patch, I will post
>> a patch later.
> 
> Please make the patch. There's too many for me to do them all.
> 
> Thanks!
> 
> -- Steve
>