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SubjectRe: perf events ring buffer memory barrier on powerpc
On Wed, Oct 30, 2013 at 04:51:16PM +0100, Peter Zijlstra wrote:
> On Wed, Oct 30, 2013 at 03:28:54PM +0200, Victor Kaplansky wrote:
> > one of the authors of Documentation/memory-barriers.txt is on cc: list ;-)
> >
> > Disclaimer: it is anyway impossible to prove lack of *any* problem.
> >
> > Having said that, lets look into an example in
> > Documentation/circular-buffers.txt:
>
> >
> > We can see that authors of the document didn't put any memory barrier
>
> Note that both documents have the same author list ;-)
>
> Anyway, I didn't know about the circular thing, I suppose I should use
> CIRC_SPACE() thing :-)

The below removes 80 bytes from ring_buffer.o of which 50 bytes are from
perf_output_begin(), it also removes 30 lines of code, so yay!

(x86_64 build)

And it appears to still work.. although I've not stressed the no-space
bits.

---
kernel/events/ring_buffer.c | 74 ++++++++++++++-------------------------------
1 file changed, 22 insertions(+), 52 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 9c2ddfbf4525..e4a51fa10595 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -12,40 +12,10 @@
#include <linux/perf_event.h>
#include <linux/vmalloc.h>
#include <linux/slab.h>
+#include <linux/circ_buf.h>

#include "internal.h"

-static bool perf_output_space(struct ring_buffer *rb, unsigned long tail,
- unsigned long offset, unsigned long head)
-{
- unsigned long sz = perf_data_size(rb);
- unsigned long mask = sz - 1;
-
- /*
- * check if user-writable
- * overwrite : over-write its own tail
- * !overwrite: buffer possibly drops events.
- */
- if (rb->overwrite)
- return true;
-
- /*
- * verify that payload is not bigger than buffer
- * otherwise masking logic may fail to detect
- * the "not enough space" condition
- */
- if ((head - offset) > sz)
- return false;
-
- offset = (offset - tail) & mask;
- head = (head - tail) & mask;
-
- if ((int)(head - offset) < 0)
- return false;
-
- return true;
-}
-
static void perf_output_wakeup(struct perf_output_handle *handle)
{
atomic_set(&handle->rb->poll, POLL_IN);
@@ -115,8 +85,7 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
rb->user_page->data_head = head;

/*
- * Now check if we missed an update, rely on the (compiler)
- * barrier in atomic_dec_and_test() to re-read rb->head.
+ * Now check if we missed an update.
*/
if (unlikely(head != local_read(&rb->head))) {
local_inc(&rb->nest);
@@ -135,7 +104,7 @@ int perf_output_begin(struct perf_output_handle *handle,
{
struct ring_buffer *rb;
unsigned long tail, offset, head;
- int have_lost;
+ int have_lost, page_shift;
struct perf_sample_data sample_data;
struct {
struct perf_event_header header;
@@ -161,7 +130,7 @@ int perf_output_begin(struct perf_output_handle *handle,
goto out;

have_lost = local_read(&rb->lost);
- if (have_lost) {
+ if (unlikely(have_lost)) {
lost_event.header.size = sizeof(lost_event);
perf_event_header__init_id(&lost_event.header, &sample_data,
event);
@@ -171,32 +140,33 @@ int perf_output_begin(struct perf_output_handle *handle,
perf_output_get_handle(handle);

do {
- /*
- * Userspace could choose to issue a mb() before updating the
- * tail pointer. So that all reads will be completed before the
- * write is issued.
- *
- * See perf_output_put_handle().
- */
tail = ACCESS_ONCE(rb->user_page->data_tail);
- smp_mb();
offset = head = local_read(&rb->head);
- head += size;
- if (unlikely(!perf_output_space(rb, tail, offset, head)))
+ if (!rb->overwrite &&
+ unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
goto fail;
+ head += size;
} while (local_cmpxchg(&rb->head, offset, head) != offset);

+ /*
+ * Userspace SHOULD issue an MB before writing the tail; see
+ * perf_output_put_handle().
+ */
+ smp_mb();
+
if (head - local_read(&rb->wakeup) > rb->watermark)
local_add(rb->watermark, &rb->wakeup);

- handle->page = offset >> (PAGE_SHIFT + page_order(rb));
- handle->page &= rb->nr_pages - 1;
- handle->size = offset & ((PAGE_SIZE << page_order(rb)) - 1);
- handle->addr = rb->data_pages[handle->page];
- handle->addr += handle->size;
- handle->size = (PAGE_SIZE << page_order(rb)) - handle->size;
+ page_shift = PAGE_SHIFT + page_order(rb);
+
+ handle->page = (offset >> page_shift) & (rb->nr_pages - 1);
+
+ offset &= page_shift - 1;
+
+ handle->addr = rb->data_pages[handle->page] + offset;
+ handle->size = (1 << page_shift) - offset;

- if (have_lost) {
+ if (unlikely(have_lost)) {
lost_event.header.type = PERF_RECORD_LOST;
lost_event.header.misc = 0;
lost_event.id = event->id;

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