[PATCH v6 05/13] ring-buffer: Add output of ring buffer meta page

Steven Rostedt
2024-06-12T23:20:06Z

Add a buffer_meta per-cpu file for the trace instance that is mapped to
boot memory. This shows the current meta-data and can be used by user
space tools to record off the current mappings to help reconstruct the
ring buffer after a reboot.

It does not expose any virtual addresses, just indexes into the sub-buffer
pages.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 77 ++++++++++++++++++++++++++++++++++++++
kernel/trace/trace.c | 30 ++++++++++++++-
kernel/trace/trace.h | 2 +
3 files changed, 107 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 0e3ed7f1cc5d..25b0e61e8c76 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -32,6 +32,8 @@
#include <asm/local64.h>
#include <asm/local.h>

+#include "trace.h"
+
/*
* The "absolute" timestamp in the buffer is only 59 bits.
* If a clock has the 5 MSBs set, it needs to be saved and
@@ -1647,6 +1649,81 @@ static void rb_range_meta_init(struct trace_buffer *buffer, int nr_pages)
}
}

+static void *rbm_start(struct seq_file *m, loff_t *pos)
+{
+ struct ring_buffer_per_cpu *cpu_buffer = m->private;
+ struct ring_buffer_meta *meta = cpu_buffer->ring_meta;
+ unsigned long val;
+
+ if (!meta)
+ return NULL;
+
+ if (*pos > meta->nr_subbufs)
+ return NULL;
+
+ val = *pos;
+ val++;
+
+ return (void *)val;
+}
+
+static void *rbm_next(struct seq_file *m, void *v, loff_t *pos)
+{
+ (*pos)++;
+
+ return rbm_start(m, pos);
+}
+
+static int rb_meta_subbuf_idx(struct ring_buffer_meta *meta, void *subbuf);
+
+static int rbm_show(struct seq_file *m, void *v)
+{
+ struct ring_buffer_per_cpu *cpu_buffer = m->private;
+ struct ring_buffer_meta *meta = cpu_buffer->ring_meta;
+ unsigned long val = (unsigned long)v;
+
+ if (val == 1) {
+ seq_printf(m, "head_buffer: %d\n",
+ rb_meta_subbuf_idx(meta, (void *)meta->head_buffer));
+ seq_printf(m, "commit_buffer: %d\n",
+ rb_meta_subbuf_idx(meta, (void *)meta->commit_buffer));
+ seq_printf(m, "subbuf_size: %d\n", meta->subbuf_size);
+ seq_printf(m, "nr_subbufs: %d\n", meta->nr_subbufs);
+ return 0;
+ }
+
+ val -= 2;
+ seq_printf(m, "buffer[%ld]: %d\n", val, meta->buffers[val]);
+
+ return 0;
+}
+
+static void rbm_stop(struct seq_file *m, void *p)
+{
+}
+
+static const struct seq_operations rb_meta_seq_ops = {
+ .start = rbm_start,
+ .next = rbm_next,
+ .show = rbm_show,
+ .stop = rbm_stop,
+};
+
+int ring_buffer_meta_seq_init(struct file *file, struct trace_buffer *buffer, int cpu)
+{
+ struct seq_file *m;
+ int ret;
+
+ ret = seq_open(file, &rb_meta_seq_ops);
+ if (ret)
+ return ret;
+
+ m = file->private_data;
+ m->private = buffer->buffers[cpu];
+
+ return 0;
+}
+
static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
long nr_pages, struct list_head *pages)
{
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ff2b504fbe00..622fe670949d 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5018,7 +5018,7 @@ static int show_traces_open(struct inode *inode, struct file *file)
return 0;
}

-static int show_traces_release(struct inode *inode, struct file *file)
+static int tracing_seq_release(struct inode *inode, struct file *file)
{
struct trace_array *tr = inode->i_private;

@@ -5059,7 +5059,7 @@ static const struct file_operations show_traces_fops = {
.open = show_traces_open,
.read = seq_read,
.llseek = seq_lseek,
- .release = show_traces_release,
+ .release = tracing_seq_release,
};

static ssize_t
@@ -6860,6 +6860,22 @@ tracing_total_entries_read(struct file *filp, char __user *ubuf,
return simple_read_from_buffer(ubuf, cnt, ppos, buf, r);
}

+static int tracing_buffer_meta_open(struct inode *inode, struct file *filp)
+{
+ struct trace_array *tr = inode->i_private;
+ int cpu = tracing_get_cpu(inode);
+ int ret;
+
+ ret = tracing_check_open_get_tr(tr);
+ if (ret)
+ return ret;
+
+ ret = ring_buffer_meta_seq_init(filp, tr->array_buffer.buffer, cpu);
+ if (ret < 0)
+ __trace_array_put(tr);
+ return ret;
+}
+
static ssize_t
tracing_free_buffer_write(struct file *filp, const char __user *ubuf,
size_t cnt, loff_t *ppos)
@@ -7436,6 +7452,13 @@ static const struct file_operations tracing_entries_fops = {
.release = tracing_release_generic_tr,
};

+static const struct file_operations tracing_buffer_meta_fops = {
+ .open = tracing_buffer_meta_open,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = tracing_seq_release,
+};
+
static const struct file_operations tracing_total_entries_fops = {
.open = tracing_open_generic_tr,
.read = tracing_total_entries_read,
@@ -8668,6 +8691,9 @@ tracing_init_tracefs_percpu(struct trace_array *tr, long cpu)
trace_create_cpu_file("buffer_size_kb", TRACE_MODE_READ, d_cpu,
tr, cpu, &tracing_entries_fops);

+ if (tr->range_addr_start)
+ trace_create_cpu_file("buffer_meta", TRACE_MODE_READ, d_cpu,
+ tr, cpu, &tracing_buffer_meta_fops);
#ifdef CONFIG_TRACER_SNAPSHOT
if (!tr->range_addr_start) {
trace_create_cpu_file("snapshot", TRACE_MODE_WRITE, d_cpu,
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 5dd48932509c..3e56d3b22212 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -645,6 +645,8 @@ trace_buffer_lock_reserve(struct trace_buffer *buffer,
unsigned long len,
unsigned int trace_ctx);

+int ring_buffer_meta_seq_init(struct file *file, struct trace_buffer *buffer, int cpu);
+
struct trace_entry *tracing_get_trace_entry(struct trace_array *tr,
struct trace_array_cpu *data);

--
2.43.0