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ftrace: make extra rcu_is_watching() validation check optional

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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 and is mostly useful for
low-level validation of ftrace subsystem invariants. For most users it
should probably be kept disabled to eliminate unnecessary runtime
overhead.

This improves BPF multi-kretprobe (relying on ftrace and rethook
infrastructure) runtime throughput by 2%, according to BPF benchmarks ([0]).

  [0] https://lore.kernel.org/bpf/CAEf4BzauQ2WKMjZdc9s0rBWa01BYbgwHN6aNDXQSHYia47pQ-w@mail.gmail.com/

Link: https://lore.kernel.org/all/20240418190909.704286-1-andrii@kernel.org/

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
This commit is contained in:
Andrii Nakryiko 2024-04-18 12:09:08 -07:00 committed by Masami Hiramatsu (Google)
parent 0dc715295d
commit b0e28a4b5b
2 changed files with 14 additions and 1 deletions
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2
include/linux/trace_recursion.h
View File

@ -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(); \

13
kernel/trace/Kconfig
View File

@ -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 execution"
depends on FUNCTION_TRACER
depends on ARCH_WANTS_NO_INSTR
help
All callbacks that attach to the function tracing have some sort of
protection against recursion. This option is only to verify that
ftrace (and other users of ftrace_test_recursion_trylock()) are not
called outside of RCU, as if they are, it can cause a race. But it
also has a noticeable overhead when enabled.
If unsure, say N
config RING_BUFFER_RECORD_RECURSION
bool "Record functions that recurse in the ring buffer"
depends on FTRACE_RECORD_RECURSION