Re: [PATCH net-next v3 2/3] ptr_ring: move r->queue[] clearing after r->consumer_head updating

[Jason Wang <jasowang@redhat.com>]

在 2021/7/1 下午8:26, Yunsheng Lin 写道:
> 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%


I think we need the number of real workloads here.

Thanks


>
> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> ---
> 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);
>   }