linux-kernel - Re: [PATCH][RFC] tracing: Enable tracepoints via module parameters

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Date:   Wed, 21 Apr 2021 16:50:30 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        "fweisbec@...il.com" <fweisbec@...il.com>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        "mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...e.hu" <mingo@...e.hu>,
        "chris@...is-wilson.co.uk" <chris@...is-wilson.co.uk>,
        "yuanhan.liu@...ux.intel.com" <yuanhan.liu@...ux.intel.com>,
        "Grumbach, Emmanuel" <emmanuel.grumbach@...el.com>
Subject: Re: [PATCH][RFC] tracing: Enable tracepoints via module parameters

On 21/04/2021 16.20, Steven Rostedt wrote:
> On Wed, 21 Apr 2021 09:30:01 +0200
> Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
> 
>> On 20/04/2021 22.32, Steven Rostedt wrote:
>>> 	/* vsnprintf() will not append null for zero-length strings */
>>> 	if (ret == 0)
>>> 		buf[0] = '\0';  
>>
>> Wrong. snprintf(buf, 16, "") will work just fine and cause a '\0' to be
>> written to buf[0]. As will snprintf(buf, 16, "%s", ""), and any other
>> case where there ends up being no characters printed.
> 
> I just cut and pasted the bpf_trace_printk() code and modified it for here.

OK, thanks for the pointer, lemme go write a patch to remove that
bogosity before it gets cargo-culted further.

> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/trace/bpf_trace.c#n404
> 
>>>
>>> #define dev_dbg(dev, fmt, ...) 					\
>>> 	do {							\
>>> 		if (trace_dev_dbg_printk_enabled())		\
>>> 			__dev_dbg(dev, fmt, ##__VA_ARGS__);	\
>>> 	} while (0)
>>>
>>> Note, the "trace_dev_dbg_printk_enabled()" is a static branch, which means
>>> it is a nop when the dev_dbg_printk tracepoint is not enabled, and is a jmp
>>> to the __dev_dbg() logic when it is enabled. It's not a conditional branch.  
>>
>> dynamic_debug has been implemented in terms of static_keys for a long
>> time. And that's a per-dev_dbg invocation static key. IIUC, the above
>> would cause every single dev_dbg in the kernel to pass through the "grab
>> a raw spin lock and do the snprintf" thing even when one is just
>> interested in the dev_dbgs inside a single driver or function.
> 
> If you want to make it per device, I'm sure three's a way. Or allocate a
> per-cpu buffer for the sprintf storage, and then you only need to disable
> interrupts. And if you make the storage 4 levels deep per CPU (like
> trace_printk does), then you only need to disable preemption and not even
> interrupts.
> 
> The above wasn't a patch submission. It was a proof of concept. Everything
> you brought up can be trivially dealt with.

I don't "want" anything. I just fail to see what advantage that proof of
concept would bring over the current dev_dbg implementation.

Rasmus