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Subject[PATCH 6/6] tracing: Write directly into splice page for trace_pipe
From: Steven Rostedt <srostedt@redhat.com>

Now that the trace_seq has a separate buffer space, we can use this
feature to write directly into the splice page, instead of writing
to the trace_seq buffer and then copying it. This has shown almost
a 4% improvement in reading trace_pipe via splice:

Average timing of reading 10megs from trace_pipe before patch:

real 0m1.619s
user 0m0.002s
sys 0m1.596s

After patch:

real 0m1.561s
user 0m0.002s
sys 0m1.535s

These were the average of five runs of reading 10megs.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 74ad344..0030d4b 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -544,25 +544,6 @@ ssize_t trace_seq_to_user(struct trace_seq *s, char __user *ubuf, size_t cnt)
return cnt;
}

-static ssize_t trace_seq_to_buffer(struct trace_seq *s, void *buf, size_t cnt)
-{
- int len;
- void *ret;
-
- if (s->len <= s->readpos)
- return -EBUSY;
-
- len = s->len - s->readpos;
- if (cnt > len)
- cnt = len;
- ret = memcpy(buf, s->buffer + s->readpos, cnt);
- if (!ret)
- return -EFAULT;
-
- s->readpos += cnt;
- return cnt;
-}
-
/*
* ftrace_max_lock is used to protect the swapping of buffers
* when taking a max snapshot. The buffers themselves are
@@ -3280,21 +3261,18 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
if (!pages[i])
break;

- rem = tracing_fill_pipe_page(rem, iter);
+ trace_seq_init(&iter->seq, page_address(pages[i]), PAGE_SIZE);

- /* Copy the data into the page, so we can start over. */
- ret = trace_seq_to_buffer(&iter->seq,
- page_address(pages[i]),
- iter->seq.len);
- if (ret < 0) {
+ rem = tracing_fill_pipe_page(rem, iter);
+ if (!iter->seq.len) {
__free_page(pages[i]);
break;
}
+
partial[i].offset = 0;
partial[i].len = iter->seq.len;
-
- trace_seq_reset(&iter->seq);
}
+ trace_seq_init(&iter->seq, iter->buffer, FTRACE_SEQ_BUFSIZE);

trace_access_unlock(iter->cpu_file);
trace_event_read_unlock();


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