tracing: Account bottom half disabled sections.

Disabling only bottom halves via local_bh_disable() disables also
preemption but this remains invisible to tracing. On a CONFIG_PREEMPT
kernel one might wonder why there is no scheduling happening despite the
N flag in the trace. The reason might be the a rcu_read_lock_bh()
section.

Add a 'b' to the tracing output if in task context with disabled bottom
halves.

Link: https://lkml.kernel.org/r/YbcbtdtC/[email protected]

Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>

Author: Sebastian Andrzej Siewior

include/linux/trace_events.h

@@ -172,6 +172,7 @@ enum trace_flag_type {
 	TRACE_FLAG_SOFTIRQ		= 0x10,
 	TRACE_FLAG_PREEMPT_RESCHED	= 0x20,
 	TRACE_FLAG_NMI			= 0x40,
+	TRACE_FLAG_BH_OFF		= 0x80,
 };
 
 #ifdef CONFIG_TRACE_IRQFLAGS_SUPPORT

kernel/trace/trace.c

@@ -2603,6 +2603,8 @@ unsigned int tracing_gen_ctx_irq_test(unsigned int irqs_status)
 		trace_flags |= TRACE_FLAG_HARDIRQ;
 	if (in_serving_softirq())
 		trace_flags |= TRACE_FLAG_SOFTIRQ;
+	if (softirq_count() >> (SOFTIRQ_SHIFT + 1))
+		trace_flags |= TRACE_FLAG_BH_OFF;
 
 	if (tif_need_resched())
 		trace_flags |= TRACE_FLAG_NEED_RESCHED;
@@ -4190,7 +4192,7 @@ unsigned long trace_total_entries(struct trace_array *tr)
 static void print_lat_help_header(struct seq_file *m)
 {
 	seq_puts(m, "#                    _------=> CPU#            \n"
-		    "#                   / _-----=> irqs-off        \n"
+		    "#                   / _-----=> irqs-off/BH-disabled\n"
 		    "#                  | / _----=> need-resched    \n"
 		    "#                  || / _---=> hardirq/softirq \n"
 		    "#                  ||| / _--=> preempt-depth   \n"
@@ -4231,7 +4233,7 @@ static void print_func_help_header_irq(struct array_buffer *buf, struct seq_file
 
 	print_event_info(buf, m);
 
-	seq_printf(m, "#                            %.*s  _-----=> irqs-off\n", prec, space);
+	seq_printf(m, "#                            %.*s  _-----=> irqs-off/BH-disabled\n", prec, space);
 	seq_printf(m, "#                            %.*s / _----=> need-resched\n", prec, space);
 	seq_printf(m, "#                            %.*s| / _---=> hardirq/softirq\n", prec, space);
 	seq_printf(m, "#                            %.*s|| / _--=> preempt-depth\n", prec, space);

kernel/trace/trace_output.c

@@ -445,14 +445,18 @@ int trace_print_lat_fmt(struct trace_seq *s, struct trace_entry *entry)
 	char irqs_off;
 	int hardirq;
 	int softirq;
+	int bh_off;
 	int nmi;
 
 	nmi = entry->flags & TRACE_FLAG_NMI;
 	hardirq = entry->flags & TRACE_FLAG_HARDIRQ;
 	softirq = entry->flags & TRACE_FLAG_SOFTIRQ;
+	bh_off = entry->flags & TRACE_FLAG_BH_OFF;
 
 	irqs_off =
+		(entry->flags & TRACE_FLAG_IRQS_OFF && bh_off) ? 'D' :
 		(entry->flags & TRACE_FLAG_IRQS_OFF) ? 'd' :
+		bh_off ? 'b' :
 		(entry->flags & TRACE_FLAG_IRQS_NOSUPPORT) ? 'X' :
 		'.';