[PATCH net-next v3 2/3] ptr_ring: move r->queue[] clearing after r->consumer_head updating
From: Yunsheng Lin <hidden>
Date: 2021-07-01 12:27:28
Also in:
linux-kselftest, lkml
Subsystem:
the rest · Maintainer:
Linus Torvalds
Currently r->queue[] clearing is done before r->consumer_head
updating, which makes the __ptr_ring_empty() returning false
positive result(the ring is non-empty, but __ptr_ring_empty()
suggest that it is empty) if the checking is done after the
r->queue clearing and before the consumer_head moving forward.
Move the r->queue[] clearing after consumer_head moving forward
to avoid the above case.
As a side effect of above change, a consumer_head checking is
avoided for the likely case, and it has noticeable performance
improvement when it is tested using the ptr_ring_test selftest
added in the previous patch.
Tested using the "perf stat -r 1000 ./ptr_ring_test -s 1000 -m 1
-N 100000000", comparing the elapsed time:
arch unpatched patched improvement
arm64 2.087205 sec 1.888224 sec +9.5%
X86 2.6538 sec 2.5422 sec +4.2%
Signed-off-by: Yunsheng Lin <redacted>
---
V3: adjust the title and comment log according to disscusion in
V2, and update performance data using "perf stat -r".
V2: Add performance data.
---
include/linux/ptr_ring.h | 25 ++++++++++++++++---------
1 file changed, 16 insertions(+), 9 deletions(-)
diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index 808f9d3..db9c282 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h@@ -261,8 +261,7 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) /* Note: we must keep consumer_head valid at all times for __ptr_ring_empty * to work correctly. */ - int consumer_head = r->consumer_head; - int head = consumer_head++; + int consumer_head = r->consumer_head + 1; /* Once we have processed enough entries invalidate them in * the ring all at once so producer can reuse their space in the ring.
@@ -271,19 +270,27 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) */ if (unlikely(consumer_head - r->consumer_tail >= r->batch || consumer_head >= r->size)) { + int tail = r->consumer_tail; + + if (unlikely(consumer_head >= r->size)) { + r->consumer_tail = 0; + WRITE_ONCE(r->consumer_head, 0); + } else { + r->consumer_tail = consumer_head; + WRITE_ONCE(r->consumer_head, consumer_head); + } + /* Zero out entries in the reverse order: this way we touch the * cache line that producer might currently be reading the last; * producer won't make progress and touch other cache lines * besides the first one until we write out all entries. */ - while (likely(head >= r->consumer_tail)) - r->queue[head--] = NULL; - r->consumer_tail = consumer_head; - } - if (unlikely(consumer_head >= r->size)) { - consumer_head = 0; - r->consumer_tail = 0; + while (likely(--consumer_head >= tail)) + r->queue[consumer_head] = NULL; + + return; } + /* matching READ_ONCE in __ptr_ring_empty for lockless tests */ WRITE_ONCE(r->consumer_head, consumer_head); }
--
2.7.4