linux-kernel - [PATCH v2] tracing: Account bottom half disabled sections.

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Date:   Mon, 13 Dec 2021 11:08:53 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH v2] 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.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
v1…v2:
 - Move output to the IRQ-off section.

 include/linux/trace_events.h | 1 +
 kernel/trace/trace.c         | 6 ++++--
 kernel/trace/trace_output.c  | 4 ++++
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 2d167ac3452c5..a2af7e1156eac 100644
--- a/include/linux/trace_events.h
+++ b/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
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 88de94da596b1..c3618dd7ecb4b 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2601,6 +2601,8 @@ 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;
@@ -4183,7 +4185,7 @@ 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"
@@ -4224,7 +4226,7 @@ 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);
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 3547e7176ff79..8aa493d25c73e 100644
--- a/kernel/trace/trace_output.c
+++ b/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' :
 		'.';
 
-- 
2.34.1