On Sun, Mar 24, 2024 at 7:38 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Fri, 22 Mar 2024 09:03:23 -0700
> Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
>
> > Introduce CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING config option to
> > control whether ftrace low-level code performs additional
> > rcu_is_watching()-based validation logic in an attempt to catch noinstr
> > violations.
> >
> > This check is expected to never be true in practice and would be best
> > controlled with extra config to let users decide if they are willing to
> > pay the price.
>
> Hmm, for me, it sounds like "WARN_ON(something) never be true in practice
> so disable it by default". I think CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING
> is OK, but tht should be set to Y by default. If you have already verified
> that your system never make it true and you want to optimize your ftrace
> path, you can manually set it to N at your own risk.
Yeah, I don't think we ever see this warning across our machines. And
sure, I can default it to Y, no problem.
>
> >
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> > include/linux/trace_recursion.h | 2 +-
> > kernel/trace/Kconfig | 13 +++++++++++++
> > 2 files changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
> > index d48cd92d2364..24ea8ac049b4 100644
> > --- a/include/linux/trace_recursion.h
> > +++ b/include/linux/trace_recursion.h
> > @@ -135,7 +135,7 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);
> > # define do_ftrace_record_recursion(ip, pip) do { } while (0)
> > #endif
> >
> > -#ifdef CONFIG_ARCH_WANTS_NO_INSTR
> > +#ifdef CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING
> > # define trace_warn_on_no_rcu(ip) \
> > ({ \
> > bool __ret = !rcu_is_watching(); \
>
> BTW, maybe we can add "unlikely" in the next "if" line?
sure, can add that as well
>
> > diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> > index 61c541c36596..19bce4e217d6 100644
> > --- a/kernel/trace/Kconfig
> > +++ b/kernel/trace/Kconfig
> > @@ -974,6 +974,19 @@ config FTRACE_RECORD_RECURSION_SIZE
> > This file can be reset, but the limit can not change in
> > size at runtime.
> >
> > +config FTRACE_VALIDATE_RCU_IS_WATCHING
> > + bool "Validate RCU is on during ftrace recursion check"
> > + depends on FUNCTION_TRACER
> > + depends on ARCH_WANTS_NO_INSTR
>
> default y
>
ok
> > + help
> > + All callbacks that attach to the function tracing have some sort
> > + of protection against recursion. This option performs additional
> > + checks to make sure RCU is on when ftrace callbacks recurse.
> > +
> > + This will add more overhead to all ftrace-based invocations.
>
> ... invocations, but keep it safe.
>
> > +
> > + If unsure, say N
>
> If unsure, say Y
>
yep, will do, thanks!
> Thank you,
>
> > +
> > config RING_BUFFER_RECORD_RECURSION
> > bool "Record functions that recurse in the ring buffer"
> > depends on FTRACE_RECORD_RECURSION
> > --
> > 2.43.0
> >
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
