ftrace: Use only the preempt version of function tracing (897f68a4) · Commits · 戴 / test

The function tracer had two different versions of function tracing. The disabling of irqs version and the preempt disable version. As function tracing in very intrusive and can cause nasty recursion issues, it has its own recursion protection. But the old method to do this was a flat layer. If it detected that a recursion was happening then it would just return without recording. This made the preempt version (much faster than the irq disabling one) not very useful, because if an interrupt were to occur after the recursion flag was set, the interrupt would not be traced at all, because every function that was traced would think it recursed on itself (due to the context it preempted setting the recursive flag). Now that we have a recursion flag for every context level, we no longer need to worry about that. We can disable preemption, set the current context recursion check bit, and go on. If an interrupt were to come along, it would check its own context bit and happily continue to trace. As the preempt version is faster than the irq disable version, there's no more reason to keep the preempt version around. And the irq disable version still had an issue with missing out on tracing NMI code. Remove the irq disable function tracer version and have the preempt disable version be the default (and only version). Before this patch we had from running: # echo function > /debug/tracing/current_tracer # for i in `seq 10`; do ./hackbench 50; done Time: 12.028 Time: 11.945 Time: 11.925 Time: 11.964 Time: 12.002 Time: 11.910 Time: 11.944 Time: 11.929 Time: 11.941 Time: 11.924 (average: 11.9512) Now we have: # echo function > /debug/tracing/current_tracer # for i in `seq 10`; do ./hackbench 50; done Time: 10.285 Time: 10.407 Time: 10.243 Time: 10.372 Time: 10.380 Time: 10.198 Time: 10.272 Time: 10.354 Time: 10.248 Time: 10.253 (average: 10.3012) a 13.8% savings! Signed-off-by:

Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_functions.c

+14 −47

Original line number	Diff line number	Diff line
		@@ -47,34 +47,6 @@ static void function_trace_start(struct trace_array *tr)
		tracing_reset_online_cpus(tr);
		}

		static void
		function_trace_call_preempt_only(unsigned long ip, unsigned long parent_ip,
		struct ftrace_ops op, struct pt_regs pt_regs)
		{
		struct trace_array *tr = func_trace;
		struct trace_array_cpu *data;
		unsigned long flags;
		long disabled;
		int cpu;
		int pc;

		if (unlikely(!ftrace_function_enabled))
		return;

		pc = preempt_count();
		preempt_disable_notrace();
		local_save_flags(flags);
		cpu = raw_smp_processor_id();
		data = tr->data[cpu];
		disabled = atomic_inc_return(&data->disabled);

		if (likely(disabled == 1))
		trace_function(tr, ip, parent_ip, flags, pc);

		atomic_dec(&data->disabled);
		preempt_enable_notrace();
		}

		/* Our option */
		enum {
		TRACE_FUNC_OPT_STACK = 0x1,
		@@ -85,34 +57,34 @@ static struct tracer_flags func_flags;
		static void
		function_trace_call(unsigned long ip, unsigned long parent_ip,
		struct ftrace_ops op, struct pt_regs pt_regs)

		{
		struct trace_array *tr = func_trace;
		struct trace_array_cpu *data;
		unsigned long flags;
		long disabled;
		unsigned int bit;
		int cpu;
		int pc;

		if (unlikely(!ftrace_function_enabled))
		return;

		/*
		* Need to use raw, since this must be called before the
		* recursive protection is performed.
		*/
		local_irq_save(flags);
		cpu = raw_smp_processor_id();
		data = tr->data[cpu];
		disabled = atomic_inc_return(&data->disabled);

		if (likely(disabled == 1)) {
		pc = preempt_count();
		preempt_disable_notrace();

		bit = trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX);
		if (bit < 0)
		goto out;

		cpu = smp_processor_id();
		data = tr->data[cpu];
		if (!atomic_read(&data->disabled)) {
		local_save_flags(flags);
		trace_function(tr, ip, parent_ip, flags, pc);
		}
		trace_clear_recursion(bit);

		atomic_dec(&data->disabled);
		local_irq_restore(flags);
		out:
		preempt_enable_notrace();
		}

		static void
		@@ -185,11 +157,6 @@ static void tracing_start_function_trace(void)
		{
		ftrace_function_enabled = 0;

		if (trace_flags & TRACE_ITER_PREEMPTONLY)
		trace_ops.func = function_trace_call_preempt_only;
		else
		trace_ops.func = function_trace_call;

		if (func_flags.val & TRACE_FUNC_OPT_STACK)
		register_ftrace_function(&trace_stack_ops);
		else

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