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: Anton Blanchard <anton@ozlabs.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index c76b2f3..136ea09 100644 (file)
--- 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);
 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
 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 802bb38..ed19413 100644 (file)
--- 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_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)
 #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);
 }
 
        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
 /**
  * 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];
 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;
 
        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();
 
        /* 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
 
 /**
  * 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)
 {
  */
 void ring_buffer_reset(struct trace_buffer *buffer)
 {

+       struct ring_buffer_per_cpu *cpu_buffer;

        int cpu;
 
        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);
 
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset);
 

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 64c5b81..4aab712 100644 (file)
--- 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;
 void tracing_reset_online_cpus(struct array_buffer *buf)
 {
        struct trace_buffer *buffer = buf->buffer;

-       int cpu;

 
        if (!buffer)
                return;
 
        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);
 
 
        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);
 }
 
        ring_buffer_record_enable(buffer);
 }