tracing: only add splice page if entries exist

The splice code allocates a page even when the ring buffer is empty.
It detects the ring buffer being empty when it it fails to copy
anything from the ring buffer into the page.

This patch adds a check to see if there is anything in the ring buffer
before allocating a page.

Thanks to logdev for letting me trace the tracer to find this.

[ Impact: speed up due to removing unnecessary allocation ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 9058240c85ca4faa525f7121abec47a43022d03b..0aeb3b93414e753adda6c80085b64b8aec577c32 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3508,7 +3508,7 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
                .spd_release    = buffer_spd_release,
        };
        struct buffer_ref *ref;
-       int size, i;
+       int entries, size, i;
        size_t ret;
 
        if (*ppos & (PAGE_SIZE - 1)) {
@@ -3523,7 +3523,9 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
                len &= PAGE_MASK;
        }
 
-       for (i = 0; i < PIPE_BUFFERS && len; i++, len -= PAGE_SIZE) {
+       entries = ring_buffer_entries_cpu(info->tr->buffer, info->cpu);
+
+       for (i = 0; i < PIPE_BUFFERS && len && entries; i++, len -= PAGE_SIZE) {
                struct page *page;
                int r;
 
@@ -3564,6 +3566,8 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
                spd.partial[i].private = (unsigned long)ref;
                spd.nr_pages++;
                *ppos += PAGE_SIZE;
+
+               entries = ring_buffer_entries_cpu(info->tr->buffer, info->cpu);
        }
 
        spd.nr_pages = i;