Messages in this thread | | | Subject | Re: [PATCH][RFC] tracing: Enable tracepoints via module parameters | From | Rasmus Villemoes <> | Date | Wed, 21 Apr 2021 16:50:30 +0200 |
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On 21/04/2021 16.20, Steven Rostedt wrote: > On Wed, 21 Apr 2021 09:30:01 +0200 > Rasmus Villemoes <linux@rasmusvillemoes.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
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