Commit 5d029b03 authored Nov 05, 2020 by Steven Rostedt (VMware)

perf/ftrace: Check for rcu_is_watching() in callback function

If a ftrace callback requires "rcu_is_watching", then it adds the
FTRACE_OPS_FL_RCU flag and it will not be called if RCU is not "watching".
But this means that it will use a trampoline when called, and this slows
down the function tracing a tad. By checking rcu_is_watching() from within
the callback, it no longer needs the RCU flag set in the ftrace_ops and it
can be safely called directly.

Link: https://lkml.kernel.org/r/20201028115613.591878956@goodmis.org
Link: https://lkml.kernel.org/r/20201106023547.711035826@goodmis.org



Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Josh  Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

parent 5d15a624

kernel/trace/trace_event_perf.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -441,6 +441,9 @@ perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
		int rctx;
		int bit;

		if (!rcu_is_watching())
		return;

		if ((unsigned long)ops->private != smp_processor_id())
		return;

		@@ -484,7 +487,6 @@ static int perf_ftrace_function_register(struct perf_event *event)
		{
		struct ftrace_ops *ops = &event->ftrace_ops;

		ops->flags = FTRACE_OPS_FL_RCU;
		ops->func = perf_ftrace_function_call;
		ops->private = (void *)(unsigned long)nr_cpu_ids;

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