Re: [PATCH v7 4/4] ftrace: Add an option for tracing console latencies

Joel Fernandes
2019-10-02T00:52:49Z

On Fri, Sep 20, 2019 at 05:22:19PM +0200, Viktor Rosendahl (BMW) wrote:
> This new trace option "console-latency" will enable the latency
> tracers to trace the console latencies. Previously this has always been
> implicitely disabled. I guess this is because they are considered
> to be well known and unavoidable.
>
> However, for some organizations it may nevertheless be desirable to
> trace them. Basically, we want to be able to tell that there are
> latencies in the system under test because someone has incorrectly
> enabled the serial console.
>
> Signed-off-by: Viktor Rosendahl (BMW) <viktor.rosendahl@gmail.com>
> ---
> include/linux/irqflags.h | 22 ++++++++++++++++++++++
> kernel/printk/printk.c | 6 ++++--
> kernel/trace/trace.h | 1 +
> kernel/trace/trace_irqsoff.c | 12 ++++++++++++
> 4 files changed, 39 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
> index 21619c92c377..3de891723331 100644
> --- a/include/linux/irqflags.h
> +++ b/include/linux/irqflags.h
> @@ -13,6 +13,7 @@
> #define _LINUX_TRACE_IRQFLAGS_H
>
> #include <linux/typecheck.h>
> +#include <linux/types.h>
> #include <asm/irqflags.h>
>
> /* Currently trace_softirqs_on/off is used only by lockdep */
> @@ -68,9 +69,30 @@ do { \
> defined(CONFIG_PREEMPT_TRACER)
> extern void stop_critical_timings(void);
> extern void start_critical_timings(void);
> + extern bool console_tracing_disabled(void);
> +
> +# define console_stop_critical_timings(flag) \
> + do { \
> + typecheck(bool, flag); \
> + flag = console_tracing_disabled(); \
> + if (flag) \
> + stop_critical_timings(); \
> + } while (0)
> +
> +# define console_start_critical_timings(flag) \
> + do { \
> + typecheck(bool, flag); \
> + if (flag) \
> + start_critical_timings(); \
> + } while (0)
> +
> #else
> # define stop_critical_timings() do { } while (0)
> # define start_critical_timings() do { } while (0)
> +# define console_stop_critical_timings(flag) \
> + do { typecheck(bool, flag); } while (0)
> +# define console_start_critical_timings(flag) \
> + do { typecheck(bool, flag); } while (0)
> #endif
>
> /*
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 1888f6a3b694..036460a7e4cd 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2359,6 +2359,7 @@ void console_unlock(void)
> static char ext_text[CONSOLE_EXT_LOG_MAX];
> static char text[LOG_LINE_MAX + PREFIX_MAX];
> unsigned long flags;
> + bool cflag;
> bool do_cond_resched, retry;
>
> if (console_suspended) {
> @@ -2459,9 +2460,10 @@ void console_unlock(void)
> */
> console_lock_spinning_enable();
>
> - stop_critical_timings(); /* don't trace print latency */
> + /* don't trace print latency if it's disabled */
> + console_stop_critical_timings(cflag);
> call_console_drivers(ext_text, ext_len, text, len);
> - start_critical_timings();
> + console_start_critical_timings(cflag);
>
> if (console_lock_spinning_disable_and_check()) {
> printk_safe_exit_irqrestore(flags);
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 4913ed1138aa..93a8b82c27e4 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1262,6 +1262,7 @@ extern int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
> C(PRINTK_MSGONLY, "printk-msg-only"), \
> C(CONTEXT_INFO, "context-info"), /* Print pid/cpu/time */ \
> C(LATENCY_FMT, "latency-format"), \
> + C(CONSOLE_LATENCY, "console-latency"), \
> C(RECORD_CMD, "record-cmd"), \
> C(RECORD_TGID, "record-tgid"), \
> C(OVERWRITE, "overwrite"), \
> diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
> index a745b0cee5d3..bc02be207a7a 100644
> --- a/kernel/trace/trace_irqsoff.c
> +++ b/kernel/trace/trace_irqsoff.c
> @@ -456,6 +456,18 @@ void stop_critical_timings(void)
> EXPORT_SYMBOL_GPL(stop_critical_timings);
> NOKPROBE_SYMBOL(stop_critical_timings);
>
> +bool console_tracing_disabled(void)
> +{
> + struct trace_array *tr = irqsoff_trace;
> + int pc = preempt_count();
> +
> + if (!preempt_trace(pc) && !irq_trace())
> + return false;

Why this condition here? Why not just make the call to
stop_critical_timings() dependent on the new flag you are adding? Please
explain it more and add some comments.

thanks,

- Joel

> +
> + return !(tr->trace_flags & TRACE_ITER_CONSOLE_LATENCY);
> +}
> +EXPORT_SYMBOL_GPL(console_tracing_disabled);
> +
> #ifdef CONFIG_FUNCTION_TRACER
> static bool function_enabled;
>
> --
> 2.17.1
>