Re: [PATCH] ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU

[Paul E. McKenney]

On Thu, Jun 25, 2020 at 03:34:03PM +1000, Nicholas Piggin wrote:
> 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@xxxxxxxxxx>
> Cc: Anton Blanchard <anton@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>

Looks plausible from an RCU viewpoint:

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  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
>