linux-kernel - [RFC][PATCH 04/11] ring-buffer: Add page_stamp to iterator for synchronization

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Date:   Tue, 17 Mar 2020 17:32:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Wu <peter@...ensteyn.nl>,
        Jonathan Corbet <corbet@....net>,
        Tom Zanussi <zanussi@...nel.org>
Subject: [RFC][PATCH 04/11] ring-buffer: Add page_stamp to iterator for synchronization

From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>

Have the ring_buffer_iter structure contain a page_stamp, such that it can
be used to see if the writer entered the page the iterator is on. When going
to a new page, the iterator will record the time stamp of that page. When
reading events, it can copy the event to an internal buffer on the iterator
(to be implemented later), then check the page's time stamp with its own to
see if the writer entered the page. If so, it will need to try to read the
event again.

Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 kernel/trace/ring_buffer.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index f57eeaa80e3e..e689bdcb53e8 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -507,6 +507,7 @@ struct ring_buffer_iter {
 	struct buffer_page		*cache_reader_page;
 	unsigned long			cache_read;
 	u64				read_stamp;
+	u64				page_stamp;
 };
 
 /**
@@ -1959,7 +1960,7 @@ static void rb_inc_iter(struct ring_buffer_iter *iter)
 	else
 		rb_inc_page(cpu_buffer, &iter->head_page);
 
-	iter->read_stamp = iter->head_page->page->time_stamp;
+	iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
 	iter->head = 0;
 }
 
@@ -3551,10 +3552,13 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
 	iter->cache_reader_page = iter->head_page;
 	iter->cache_read = cpu_buffer->read;
 
-	if (iter->head)
+	if (iter->head) {
 		iter->read_stamp = cpu_buffer->read_stamp;
-	else
+		iter->page_stamp = cpu_buffer->reader_page->page->time_stamp;
+	} else {
 		iter->read_stamp = iter->head_page->page->time_stamp;
+		iter->page_stamp = iter->read_stamp;
+	}
 }
 
 /**
-- 
2.25.1