[PATCH 3/5] ring-buffer: read page interface

Steven Rostedt
2008-12-02T20:35:57Z

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From: Steven Rostedt <srostedt@redhat.com>

Impact: new API to ring buffer

This patch adds a new interface into the ring buffer that allows a
page to be read from the ring buffer on a given CPU. For every page
read, one must also be given to allow for a "swap" of the pages.

rpage = ring_buffer_alloc_read_page(buffer);
if (!rpage)
goto err;
ret = ring_buffer_read_page(buffer, &rpage, cpu, full);
if (!ret)
goto empty;
process_page(rpage);
ring_buffer_free_read_page(rpage);

The caller of these functions must handle any waits that are
needed to wait for new data. The ring_buffer_read_page will simply
return 0 if there is no data, or if "full" is set and the writer
is still on the current page.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
---
include/linux/ring_buffer.h | 5 ++
kernel/trace/ring_buffer.c | 166 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 171 insertions(+), 0 deletions(-)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index 3bb87a7..1a350a8 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -124,6 +124,11 @@ void tracing_on(void);
void tracing_off(void);
void tracing_off_permanent(void);

+void *ring_buffer_alloc_read_page(struct ring_buffer *buffer);
+void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data);
+int ring_buffer_read_page(struct ring_buffer *buffer,
+ void **data_page, int cpu, int full);
+
enum ring_buffer_flags {
RB_FL_OVERWRITE = 1 << 0,
};
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 8619c53..50b74d3 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -687,6 +687,12 @@ static inline int rb_null_event(struct ring_buffer_event *event)
return event->type == RINGBUF_TYPE_PADDING;
}

+static inline void *
+__rb_data_page_index(struct buffer_data_page *page, unsigned index)
+{
+ return page->data + index;
+}
+
static inline void *__rb_page_index(struct buffer_page *page, unsigned index)
{
return page->page->data + index;
@@ -2232,6 +2238,166 @@ int ring_buffer_swap_cpu(struct ring_buffer *buffer_a,
return 0;
}

+static void rb_remove_entries(struct ring_buffer_per_cpu *cpu_buffer,
+ struct buffer_data_page *page)
+{
+ struct ring_buffer_event *event;
+ unsigned long head;
+
+ __raw_spin_lock(&cpu_buffer->lock);
+ for (head = 0; head < local_read(&page->commit);
+ head += rb_event_length(event)) {
+
+ event = __rb_data_page_index(page, head);
+ if (RB_WARN_ON(cpu_buffer, rb_null_event(event)))
+ return;
+ /* Only count data entries */
+ if (event->type != RINGBUF_TYPE_DATA)
+ continue;
+ cpu_buffer->entries--;
+ }
+ __raw_spin_unlock(&cpu_buffer->lock);
+}
+
+/**
+ * ring_buffer_alloc_read_page - allocate a page to read from buffer
+ * @buffer: the buffer to allocate for.
+ *
+ * This function is used in conjunction with ring_buffer_read_page.
+ * When reading a full page from the ring buffer, these functions
+ * can be used to speed up the process. The calling function should
+ * allocate a few pages first with this function. Then when it
+ * needs to get pages from the ring buffer, it passes the result
+ * of this function into ring_buffer_read_page, which will swap
+ * the page that was allocated, with the read page of the buffer.
+ *
+ * Returns:
+ * The page allocated, or NULL on error.
+ */
+void *ring_buffer_alloc_read_page(struct ring_buffer *buffer)
+{
+ unsigned long addr;
+ struct buffer_data_page *page;
+
+ addr = __get_free_page(GFP_KERNEL);
+ if (!addr)
+ return NULL;
+
+ page = (void *)addr;
+
+ return page;
+}
+
+/**
+ * ring_buffer_free_read_page - free an allocated read page
+ * @buffer: the buffer the page was allocate for
+ * @data: the page to free
+ *
+ * Free a page allocated from ring_buffer_alloc_read_page.
+ */
+void ring_buffer_free_read_page(struct ring_buffer *buffer, void *data)
+{
+ free_page((unsigned long)data);
+}
+
+/**
+ * ring_buffer_read_page - extract a page from the ring buffer
+ * @buffer: buffer to extract from
+ * @data_page: the page to use allocated from ring_buffer_alloc_read_page
+ * @cpu: the cpu of the buffer to extract
+ * @full: should the extraction only happen when the page is full.
+ *
+ * This function will pull out a page from the ring buffer and consume it.
+ * @data_page must be the address of the variable that was returned
+ * from ring_buffer_alloc_read_page. This is because the page might be used
+ * to swap with a page in the ring buffer.
+ *
+ * for example:
+ * rpage = ring_buffer_alloc_page(buffer);
+ * if (!rpage)
+ * return error;
+ * ret = ring_buffer_read_page(buffer, &rpage, cpu, 0);
+ * if (ret)
+ * process_page(rpage);
+ *
+ * When @full is set, the function will not return true unless
+ * the writer is off the reader page.
+ *
+ * Note: it is up to the calling functions to handle sleeps and wakeups.
+ * The ring buffer can be used anywhere in the kernel and can not
+ * blindly call wake_up. The layer that uses the ring buffer must be
+ * responsible for that.
+ *
+ * Returns:
+ * 1 if data has been transferred
+ * 0 if no data has been transferred.
+ */
+int ring_buffer_read_page(struct ring_buffer *buffer,
+ void **data_page, int cpu, int full)
+{
+ struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
+ struct ring_buffer_event *event;
+ struct buffer_data_page *page;
+ unsigned long flags;
+ int ret = 0;
+
+ if (!data_page)
+ return 0;
+
+ page = *data_page;
+ if (!page)
+ return 0;
+
+ spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
+
+ /*
+ * rb_buffer_peek will get the next ring buffer if
+ * the current reader page is empty.
+ */
+ event = rb_buffer_peek(buffer, cpu, NULL);
+ if (!event)
+ goto out;
+
+ /* check for data */
+ if (!local_read(&cpu_buffer->reader_page->page->commit))
+ goto out;
+ /*
+ * If the writer is already off of the read page, then simply
+ * switch the read page with the given page. Otherwise
+ * we need to copy the data from the reader to the writer.
+ */
+ if (cpu_buffer->reader_page == cpu_buffer->commit_page) {
+ unsigned int read = cpu_buffer->reader_page->read;
+
+ if (full)
+ goto out;
+ /* The writer is still on the reader page, we must copy */
+ page = cpu_buffer->reader_page->page;
+ memcpy(page->data,
+ cpu_buffer->reader_page->page->data + read,
+ local_read(&page->commit) - read);
+
+ /* consume what was read */
+ cpu_buffer->reader_page += read;
+
+ } else {
+ /* swap the pages */
+ rb_init_page(page);
+ page = cpu_buffer->reader_page->page;
+ cpu_buffer->reader_page->page = *data_page;
+ cpu_buffer->reader_page->read = 0;
+ *data_page = page;
+ }
+ ret = 1;
+
+ /* update the entry counter */
+ rb_remove_entries(cpu_buffer, page);
+ out:
+ spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
+
+ return ret;
+}
+
static ssize_t
rb_simple_read(struct file *filp, char __user *ubuf,
size_t cnt, loff_t *ppos)
--
1.5.6.5

--