Commit b23d7a5f authored by Nicholas Piggin's avatar Nicholas Piggin Committed by Steven Rostedt (VMware)
Browse files

ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU 

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.

Link: https://lkml.kernel.org/r/20200625053403.2386972-1-npiggin@gmail.com



Tested-by: default avatarAnton Blanchard <anton@ozlabs.org>
Acked-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarNicholas Piggin <npiggin@gmail.com>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent 10464b4a

include/linux/ring_buffer.h

+1 −0
Original line number Diff line number Diff line
@@ -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

kernel/trace/ring_buffer.c

+71 −14
Original line number Diff line number Diff line
@@ -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)
@@ -4790,6 +4793,26 @@ 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
@@ -4798,7 +4821,6 @@ 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;
@@ -4809,24 +4831,42 @@ 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();



	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
@@ -4834,10 +4874,27 @@ 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);

kernel/trace/trace.c

+1 −3
Original line number Diff line number Diff line
@@ -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);

}

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