tracing: Add trace_dump_stack()
I've been asked a few times about how to find out what is calling some location in the kernel. One way is to use dynamic function tracing and implement the func_stack_trace. But this only finds out who is calling a particular function. It does not tell you who is calling that function and entering a specific if conditional. I have myself implemented a quick version of trace_dump_stack() for this purpose a few times, and just needed it now. This is when I realized that this would be a good tool to have in the kernel like trace_printk(). Using trace_dump_stack() is similar to dump_stack() except that it writes to the trace buffer instead and can be used in critical locations. For example: @@ -5485,8 +5485,12 @@ need_resched_nonpreemptible: if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) { if (unlikely(signal_pending_state(prev->state, prev))) prev->state = TASK_RUNNING; - else + else { deactivate_task(rq, prev, 1); + trace_printk("Deactivating task %s:%d\n", + prev->comm, prev->pid); + trace_dump_stack(); + } switch_count = &prev->nvcsw; } Produces: <...>-3249 [001] 296.105269: schedule: Deactivating task ntpd:3249 <...>-3249 [001] 296.105270: <stack trace> => schedule => schedule_hrtimeout_range => poll_schedule_timeout => do_select => core_sys_select => sys_select => system_call_fastpath Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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@ -492,6 +492,8 @@ extern int
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__trace_printk(unsigned long ip, const char *fmt, ...)
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__attribute__ ((format (printf, 2, 3)));
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extern void trace_dump_stack(void);
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/*
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* The double __builtin_constant_p is because gcc will give us an error
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* if we try to allocate the static variable to fmt if it is not a
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@ -1151,6 +1151,22 @@ void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,
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__ftrace_trace_stack(tr->buffer, flags, skip, pc);
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}
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/**
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* trace_dump_stack - record a stack back trace in the trace buffer
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*/
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void trace_dump_stack(void)
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{
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unsigned long flags;
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if (tracing_disabled || tracing_selftest_running)
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return 0;
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local_save_flags(flags);
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/* skipping 3 traces, seems to get us at the caller of this function */
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__ftrace_trace_stack(global_trace.buffer, flags, 3, preempt_count());
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}
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void
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ftrace_trace_userstack(struct ring_buffer *buffer, unsigned long flags, int pc)
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{
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