From: Nicholas Piggin <npiggin@gmail.com>
Date: Thu, 25 Jun 2020 15:34:03 +1000
Subject: ring-buffer: speed up buffer resets by avoiding synchronize_rcu for
 each CPU
Git-commit: b23d7a5f4a07af02343cdd28fe1f7488bac3afda
Patch-mainline: v5.9-rc1
References: CVE-2020-27825 bsc#1179960

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>
Acked-by: Miroslav Benes <mbenes@suse.cz>
---
 include/linux/ring_buffer.h |    1 
 kernel/trace/ring_buffer.c  |   85 ++++++++++++++++++++++++++++++++++++--------
 kernel/trace/trace.c        |    4 --
 3 files changed, 73 insertions(+), 17 deletions(-)

--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -140,6 +140,7 @@ int ring_buffer_iter_empty(struct ring_b
 unsigned long ring_buffer_size(struct ring_buffer *buffer, int cpu);
 
 void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu);
+void ring_buffer_reset_online_cpus(struct ring_buffer *buffer);
 void ring_buffer_reset(struct ring_buffer *buffer);
 
 #ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -272,6 +272,9 @@ 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)
@@ -4219,6 +4222,26 @@ rb_reset_cpu(struct ring_buffer_per_cpu
 	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
@@ -4227,7 +4250,6 @@ rb_reset_cpu(struct ring_buffer_per_cpu
 void ring_buffer_reset_cpu(struct ring_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;
@@ -4238,24 +4260,42 @@ void ring_buffer_reset_cpu(struct ring_b
 	/* Make sure all commits have finished */
 	synchronize_sched();
 
-	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 ring_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
@@ -4263,10 +4303,27 @@ EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu)
  */
 void ring_buffer_reset(struct ring_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);
 
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1681,7 +1681,6 @@ void tracing_reset(struct trace_buffer *
 void tracing_reset_online_cpus(struct trace_buffer *buf)
 {
 	struct ring_buffer *buffer = buf->buffer;
-	int cpu;
 
 	if (!buffer)
 		return;
@@ -1693,8 +1692,7 @@ void tracing_reset_online_cpus(struct tr
 
 	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);
 }