Messages in this thread | | | From | John Ogness <> | Subject | Re: [PATCH] dynamic debug: allow printing to trace event | Date | Thu, 23 Jul 2020 16:08:44 +0206 |
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On 2020-07-22, Steven Rostedt <rostedt@goodmis.org> wrote: >>> +static void dynamic_printk(unsigned int flags, const char *fmt, ...) >>> +{ >>> + if (flags & _DPRINTK_FLAGS_TRACE) { >>> + va_list args; >>> + >>> + va_start(args, fmt); >>> + /* >>> + * All callers include the KERN_DEBUG prefix to keep the >>> + * vprintk case simple; strip it out for tracing. >>> + */ >>> + dynamic_trace(fmt + strlen(KERN_DEBUG), args); >> >> Do we really need a separate tracing event for this? Why not just: >> >> ftrace_vprintk(fmt + strlen(KERN_DEBUG), args); > > It must be an event, one that can be enabled or disabled separately > from trace_printk(). > > If you are asking if this could be something like trace_printk(), > which ftrace_vprintk() is. The reason for that nasty banner when > people use trace_printk() is to keep developers from using it as their > personal debugging tool in production. > > A trace_printk() can not be discretely disabled. It's either totally > on, or totally off. And since it is used for debugging, if there's > trace_printk()s all over the kernel, you will have to deal with the > noise of everyone else's trace_printk(), making trace_printk() > useless.
I understand and agree with your concern about trace_printk(). But it seems to me that trace_printk() via pr_debug() should be OK because there is discrete control per message implemented. Yes, more code is necessary to distinguish between the two, such as letting dynamic_printk use an internal function that does not trigger a splat. But I think that is reasonable.
For me a trace event represents a specific point in the kernel code. But this new printk trace event, instead, represents general log redirection. I do not oppose it, but it feels like a hack to me. In contrast, simply setting a dynamic printk flag to write the message string to the trace buffer (without also activating some pseudo trace event) feels more natural.
Just sharing my thoughts, as requested.
John Ogness
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