Messages in this thread | | | Date | Tue, 3 May 2022 12:15:46 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH -printk] printk, tracing: fix console tracepoint |
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On Tue, 3 May 2022 09:38:44 +0200 Marco Elver <elver@google.com> wrote:
> The original intent of the 'console' tracepoint per 95100358491a > ("printk/tracing: Add console output tracing") had been to "[...] record > any printk messages into the trace, regardless of the current console > loglevel. This can help correlate (existing) printk debugging with other > tracing." > > Petr points out [1] that calling trace_console_rcuidle() in > call_console_driver() had been the wrong thing for a while, because > "printk() always used console_trylock() and the message was flushed to > the console only when the trylock succeeded. And it was always deferred > in NMI or when printed via printk_deferred()." > > With 09c5ba0aa2fc ("printk: add kthread console printers"), things only > got worse, and calls to call_console_driver() no longer happen with > typical printk() calls but always appear deferred [2]. > > As such, the tracepoint can no longer serve its purpose to clearly > correlate printk() calls and other tracing, as well as breaks usecases > that expect every printk() call to result in a callback of the console > tracepoint. Notably, the KFENCE and KCSAN test suites, which want to > capture console output and assume a printk() immediately gives us a > callback to the console tracepoint. > > Fix the console tracepoint by moving it into printk_sprint() [3]. > > One notable difference is that by moving tracing into printk_sprint(), > the 'text' will no longer include the "header" (loglevel and timestamp), > but only the raw message. Arguably this is less of a problem now that > the console tracepoint happens on the printk() call and isn't delayed. >
I'm OK with this change, but I don't know everyone that uses the trace printk feature. I am worried that this could cause regressions in people's workloads.
I'd like to hear more feedback from others, but for me:
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
-- Steve
> Link: https://lore.kernel.org/all/Ym+WqKStCg%2FEHfh3@alley/ [1] > Link: https://lore.kernel.org/all/CA+G9fYu2kS0wR4WqMRsj2rePKV9XLgOU1PiXnMvpT+Z=c2ucHA@mail.gmail.com/ [2] > Link: https://lore.kernel.org/all/87fslup9dx.fsf@jogness.linutronix.de/ [3] > Reported-by: Linux Kernel Functional Testing <lkft@linaro.org> > Signed-off-by: Marco Elver <elver@google.com> > Cc: John Ogness <john.ogness@linutronix.de> > Cc: Petr Mladek <pmladek@suse.com> > --- > kernel/printk/printk.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index f66d6e72a642..a3e1035929b0 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -2064,8 +2064,6 @@ static void call_console_driver(struct console *con, const char *text, size_t le > { > size_t dropped_len; > > - trace_console_rcuidle(text, len); > - > if (con->dropped && dropped_text) { > dropped_len = snprintf(dropped_text, DROPPED_TEXT_MAX, > "** %lu printk messages dropped **\n", > @@ -2240,6 +2238,8 @@ static u16 printk_sprint(char *text, u16 size, int facility, > } > } > > + trace_console_rcuidle(text, text_len); > + > return text_len; > } >
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