慢慢欣赏 ftrace （一）找到解析function对应字段的函数

分析ftrace的时候，有时会对其字段部分不了解，有些字段解析的不清晰，例如D和~

# cat /sys/kernel/debug/tracing/trace      
# tracer: function
#
# entries-in-buffer/entries-written: 0/23029303390   #P:8
#
#                                _-----=> irqs-off
#                               / _----=> need-resched
#                              | / _---=> hardirq/softirq
#                              || / _--=> preempt-depth
#                              ||| /     delay
#           TASK-PID     CPU#  ||||   TIMESTAMP  FUNCTION
#              | |         |   ||||      |         |
   kworker/u16:1-503     [003] D.~2  3037.587262: irq_find_mapping <-gic_handle_irq

对于5.10内核，我们找到其对应函数 trace_print_context，该函数可以仔细研读

trace_print_context
	=>trace_seq_printf(s, "%16s-%-7d ", comm, entry->pid);		// 打印线程名称和PID
	=>trace_seq_printf(s, "[%03d] ", iter->cpu);			// 打印线程所在CORE信息
	=>trace_print_lat_fmt(s, entry);				// 打印调度信息
	=>trace_seq_printf(s, " %5lu.%06lu: ", secs, usec_rem);		// 打印时间信息

继续研读 trace_print_lat_fmt

/**
 * trace_print_lat_fmt - print the irq, preempt and lockdep fields
 * @s: trace seq struct to write to
 * @entry: The trace entry field from the ring buffer
 *
 * Prints the generic fields of irqs off, in hard or softirq, preempt
 * count.
 */
int trace_print_lat_fmt(struct trace_seq *s, struct trace_entry *entry)
{
	char hardsoft_irq;
	char need_resched;
	char irqs_off;
	int hardirq;
	int softirq;
	int nmi;

	nmi = entry->flags & TRACE_FLAG_NMI;
	hardirq = entry->flags & TRACE_FLAG_HARDIRQ;
	softirq = entry->flags & TRACE_FLAG_SOFTIRQ;

	irqs_off =
		(entry->flags & TRACE_FLAG_IRQS_OFF) ? 'd' :
		(entry->flags & TRACE_FLAG_IRQS_NOSUPPORT) ? 'X' :
		'.';

	switch (entry->flags & (TRACE_FLAG_NEED_RESCHED |
				TRACE_FLAG_PREEMPT_RESCHED)) {
	case TRACE_FLAG_NEED_RESCHED | TRACE_FLAG_PREEMPT_RESCHED:
		need_resched = 'N';
		break;
	case TRACE_FLAG_NEED_RESCHED:
		need_resched = 'n';
		break;
	case TRACE_FLAG_PREEMPT_RESCHED:
		need_resched = 'p';
		break;
	default:
		need_resched = '.';
		break;
	}

	hardsoft_irq =
		(nmi && hardirq)     ? 'Z' :
		nmi                  ? 'z' :
		(hardirq && softirq) ? 'H' :
		hardirq              ? 'h' :
		softirq              ? 's' :
		                       '.' ;

	trace_seq_printf(s, "%c%c%c",
			 irqs_off, need_resched, hardsoft_irq);

	if (entry->preempt_count)
		trace_seq_printf(s, "%x", entry->preempt_count);
	else
		trace_seq_putc(s, '.');

	return !trace_seq_has_overflowed(s);
}