tracing: Set actual size after ring buffer resize

Currently we can resize trace ringbuffer by writing a value into file
'buffer_size_kb', then by reading the file, we get the value that is
usually what we wrote. However, this value may be not actual size of
trace ring buffer because of the round up when doing resize in kernel,
and the actual size would be more useful.

Link: https://lore.kernel.org/linux-trace-kernel/20230705002705.576633-1-zhengyejian1@huawei.com

Cc: <mhiramat@kernel.org>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -6286,6 +6286,15 @@ static void set_buffer_entries(struct array_buffer *buf, unsigned long val)
 		per_cpu_ptr(buf->data, cpu)->entries = val;
 }
 
+static void update_buffer_entries(struct array_buffer *buf, int cpu)
+{
+	if (cpu == RING_BUFFER_ALL_CPUS) {
+		set_buffer_entries(buf, ring_buffer_size(buf->buffer, 0));
+	} else {
+		per_cpu_ptr(buf->data, cpu)->entries = ring_buffer_size(buf->buffer, cpu);
+	}
+}
+
 #ifdef CONFIG_TRACER_MAX_TRACE
 /* resize @tr's buffer to the size of @size_tr's entries */
 static int resize_buffer_duplicate_size(struct array_buffer *trace_buf,
@@ -6364,18 +6373,12 @@ static int __tracing_resize_ring_buffer(struct trace_array *tr,
 		return ret;
 	}
 
-	if (cpu == RING_BUFFER_ALL_CPUS)
-		set_buffer_entries(&tr->max_buffer, size);
-	else
-		per_cpu_ptr(tr->max_buffer.data, cpu)->entries = size;
+	update_buffer_entries(&tr->max_buffer, cpu);
 
  out:
 #endif /* CONFIG_TRACER_MAX_TRACE */
 
-	if (cpu == RING_BUFFER_ALL_CPUS)
-		set_buffer_entries(&tr->array_buffer, size);
-	else
-		per_cpu_ptr(tr->array_buffer.data, cpu)->entries = size;
+	update_buffer_entries(&tr->array_buffer, cpu);
 
 	return ret;
 }