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Subject[RFC PATCH] trace: 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 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@linutronix.de>
---
include/linux/trace_events.h | 1 +
kernel/trace/trace.c | 6 ++++--
kernel/trace/trace_output.c | 3 +++
3 files changed, 8 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..dca48d556ee5a 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;
@@ -4185,7 +4187,7 @@ static void print_lat_help_header(struct seq_file *m)
seq_puts(m, "# _------=> CPU# \n"
"# / _-----=> irqs-off \n"
"# | / _----=> need-resched \n"
- "# || / _---=> hardirq/softirq \n"
+ "# || / _---=> hardirq/softirq/BH-disables\n"
"# ||| / _--=> preempt-depth \n"
"# |||| / _-=> migrate-disable \n"
"# ||||| / delay \n"
@@ -4226,7 +4228,7 @@ static void print_func_help_header_irq(struct array_buffer *buf, struct seq_file

seq_printf(m, "# %.*s _-----=> irqs-off\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| / _---=> hardirq/softirq/BH-disabled\n", prec, space);
seq_printf(m, "# %.*s|| / _--=> preempt-depth\n", prec, space);
seq_printf(m, "# %.*s||| / _-=> migrate-disable\n", prec, space);
seq_printf(m, "# %.*s|||| / delay\n", prec, space);
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 3547e7176ff79..6be644d35ec30 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -445,11 +445,13 @@ 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) ? 'd' :
@@ -478,6 +480,7 @@ int trace_print_lat_fmt(struct trace_seq *s, struct trace_entry *entry)
(hardirq && softirq) ? 'H' :
hardirq ? 'h' :
softirq ? 's' :
+ bh_off ? 'b' :
'.' ;

trace_seq_printf(s, "%c%c%c",
--
2.34.1
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