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Subject[PATCH] ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU
Date
On a 144 thread system, `perf ftrace` takes about 20 seconds to start
up, due to calling synchronize_rcu() for each CPU.

cat /proc/108560/stack
0xc0003e7eb336f470
__switch_to+0x2e0/0x480
__wait_rcu_gp+0x20c/0x220
synchronize_rcu+0x9c/0xc0
ring_buffer_reset_cpu+0x88/0x2e0
tracing_reset_online_cpus+0x84/0xe0
tracing_open+0x1d4/0x1f0

On a system with 10x more threads, it starts to become an annoyance.

Batch these up so we disable all the per-cpu buffers first, then
synchronize_rcu() once, then reset each of the buffers. This brings
the time down to about 0.5s.

Cc: Paul McKenney <paulmck@kernel.org>
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
include/linux/ring_buffer.h | 1 +
kernel/trace/ring_buffer.c | 85 +++++++++++++++++++++++++++++++------
kernel/trace/trace.c | 4 +-
3 files changed, 73 insertions(+), 17 deletions(-)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index c76b2f3b3ac4..136ea0997e6d 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -143,6 +143,7 @@ bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter);
unsigned long ring_buffer_size(struct trace_buffer *buffer, int cpu);

void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu);
+void ring_buffer_reset_online_cpus(struct trace_buffer *buffer);
void ring_buffer_reset(struct trace_buffer *buffer);

#ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index b8e1ca48be50..3f1fd02bd14a 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -270,6 +270,9 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data);
#define for_each_buffer_cpu(buffer, cpu) \
for_each_cpu(cpu, buffer->cpumask)

+#define for_each_online_buffer_cpu(buffer, cpu) \
+ for_each_cpu_and(cpu, buffer->cpumask, cpu_online_mask)
+
#define TS_SHIFT 27
#define TS_MASK ((1ULL << TS_SHIFT) - 1)
#define TS_DELTA_TEST (~TS_MASK)
@@ -4484,6 +4487,26 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
rb_head_page_activate(cpu_buffer);
}

+/* Must have disabled the cpu buffer then done a synchronize_rcu */
+static void reset_disabled_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
+{
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+
+ if (RB_WARN_ON(cpu_buffer, local_read(&cpu_buffer->committing)))
+ goto out;
+
+ arch_spin_lock(&cpu_buffer->lock);
+
+ rb_reset_cpu(cpu_buffer);
+
+ arch_spin_unlock(&cpu_buffer->lock);
+
+ out:
+ raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+}
+
/**
* ring_buffer_reset_cpu - reset a ring buffer per CPU buffer
* @buffer: The ring buffer to reset a per cpu buffer of
@@ -4492,7 +4515,6 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
{
struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
- unsigned long flags;

if (!cpumask_test_cpu(cpu, buffer->cpumask))
return;
@@ -4503,24 +4525,42 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
/* Make sure all commits have finished */
synchronize_rcu();

- raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+ reset_disabled_cpu_buffer(cpu_buffer);

- if (RB_WARN_ON(cpu_buffer, local_read(&cpu_buffer->committing)))
- goto out;
+ atomic_dec(&cpu_buffer->record_disabled);
+ atomic_dec(&cpu_buffer->resize_disabled);
+}
+EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);

- arch_spin_lock(&cpu_buffer->lock);
+/**
+ * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer
+ * @buffer: The ring buffer to reset a per cpu buffer of
+ * @cpu: The CPU buffer to be reset
+ */
+void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
+{
+ struct ring_buffer_per_cpu *cpu_buffer;
+ int cpu;

- rb_reset_cpu(cpu_buffer);
+ for_each_online_buffer_cpu(buffer, cpu) {
+ cpu_buffer = buffer->buffers[cpu];

- arch_spin_unlock(&cpu_buffer->lock);
+ atomic_inc(&cpu_buffer->resize_disabled);
+ atomic_inc(&cpu_buffer->record_disabled);
+ }

- out:
- raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+ /* Make sure all commits have finished */
+ synchronize_rcu();

- atomic_dec(&cpu_buffer->record_disabled);
- atomic_dec(&cpu_buffer->resize_disabled);
+ for_each_online_buffer_cpu(buffer, cpu) {
+ cpu_buffer = buffer->buffers[cpu];
+
+ reset_disabled_cpu_buffer(cpu_buffer);
+
+ atomic_dec(&cpu_buffer->record_disabled);
+ atomic_dec(&cpu_buffer->resize_disabled);
+ }
}
-EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);

/**
* ring_buffer_reset - reset a ring buffer
@@ -4528,10 +4568,27 @@ EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
*/
void ring_buffer_reset(struct trace_buffer *buffer)
{
+ struct ring_buffer_per_cpu *cpu_buffer;
int cpu;

- for_each_buffer_cpu(buffer, cpu)
- ring_buffer_reset_cpu(buffer, cpu);
+ for_each_buffer_cpu(buffer, cpu) {
+ cpu_buffer = buffer->buffers[cpu];
+
+ atomic_inc(&cpu_buffer->resize_disabled);
+ atomic_inc(&cpu_buffer->record_disabled);
+ }
+
+ /* Make sure all commits have finished */
+ synchronize_rcu();
+
+ for_each_buffer_cpu(buffer, cpu) {
+ cpu_buffer = buffer->buffers[cpu];
+
+ reset_disabled_cpu_buffer(cpu_buffer);
+
+ atomic_dec(&cpu_buffer->record_disabled);
+ atomic_dec(&cpu_buffer->resize_disabled);
+ }
}
EXPORT_SYMBOL_GPL(ring_buffer_reset);

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ec44b0e2a19c..9a26a1c875ae 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2003,7 +2003,6 @@ static void tracing_reset_cpu(struct array_buffer *buf, int cpu)
void tracing_reset_online_cpus(struct array_buffer *buf)
{
struct trace_buffer *buffer = buf->buffer;
- int cpu;

if (!buffer)
return;
@@ -2015,8 +2014,7 @@ void tracing_reset_online_cpus(struct array_buffer *buf)

buf->time_start = buffer_ftrace_now(buf, buf->cpu);

- for_each_online_cpu(cpu)
- ring_buffer_reset_cpu(buffer, cpu);
+ ring_buffer_reset_online_cpus(buffer);

ring_buffer_record_enable(buffer);
}
--
2.23.0
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