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From: Pavel Begunkov <[email protected]>
To: Hao Xu <[email protected]>, Jens Axboe <[email protected]>
Cc: [email protected], Joseph Qi <[email protected]>
Subject: Re: [PATCH 1/8] io-wq: code clean for io_wq_add_work_after()
Date: Tue, 28 Sep 2021 12:08:24 +0100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 9/27/21 7:17 AM, Hao Xu wrote:
> Remove a local variable.

It's there to help alias analysis, which usually can't do anything
with pointer heavy logic. Compare ASMs below, before and after
respectively:
	testq	%rax, %rax	# next

replaced with
	cmpq	$0, (%rdi)	#, node_2(D)->next

One extra memory dereference and a bigger binary


=====================================================

wq_list_add_after:
# fs/io_uring.c:271: 	struct io_wq_work_node *next = pos->next;
	movq	(%rsi), %rax	# pos_3(D)->next, next
# fs/io_uring.c:273: 	pos->next = node;
	movq	%rdi, (%rsi)	# node, pos_3(D)->next
# fs/io_uring.c:275: 	if (!next)
	testq	%rax, %rax	# next
# fs/io_uring.c:274: 	node->next = next;
	movq	%rax, (%rdi)	# next, node_5(D)->next
# fs/io_uring.c:275: 	if (!next)
	je	.L5927	#,
	ret	
.L5927:
# fs/io_uring.c:276: 		list->last = node;
	movq	%rdi, 8(%rdx)	# node, list_8(D)->last
	ret	

=====================================================

wq_list_add_after:
# fs/io-wq.h:48: 	node->next = pos->next;
	movq	(%rsi), %rax	# pos_3(D)->next, _5
# fs/io-wq.h:48: 	node->next = pos->next;
	movq	%rax, (%rdi)	# _5, node_2(D)->next
# fs/io-wq.h:49: 	pos->next = node;
	movq	%rdi, (%rsi)	# node, pos_3(D)->next
# fs/io-wq.h:50: 	if (!node->next)
	cmpq	$0, (%rdi)	#, node_2(D)->next
	je	.L5924	#,
	ret	
.L5924:
# fs/io-wq.h:51: 		list->last = node;
	movq	%rdi, 8(%rdx)	# node, list_4(D)->last
	ret	


> 
> Signed-off-by: Hao Xu <[email protected]>
> ---
>  fs/io-wq.h | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/io-wq.h b/fs/io-wq.h
> index bf5c4c533760..8369a51b65c0 100644
> --- a/fs/io-wq.h
> +++ b/fs/io-wq.h
> @@ -33,11 +33,9 @@ static inline void wq_list_add_after(struct io_wq_work_node *node,
>  				     struct io_wq_work_node *pos,
>  				     struct io_wq_work_list *list)
>  {
> -	struct io_wq_work_node *next = pos->next;
> -
> +	node->next = pos->next;
>  	pos->next = node;
> -	node->next = next;
> -	if (!next)
> +	if (!node->next)
>  		list->last = node;
>  }
>  
> 

-- 
Pavel Begunkov

  reply	other threads:[~2021-09-28 11:09 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-27  6:17 [PATCH 0/6] task_work optimization Hao Xu
2021-09-27  6:17 ` [PATCH 1/8] io-wq: code clean for io_wq_add_work_after() Hao Xu
2021-09-28 11:08   ` Pavel Begunkov [this message]
2021-09-29  7:36     ` Hao Xu
2021-09-29 11:23       ` Pavel Begunkov
2021-09-27  6:17 ` [PATCH 2/8] io-wq: add helper to merge two wq_lists Hao Xu
2021-09-27 10:17   ` Hao Xu
2021-09-28 11:10   ` Pavel Begunkov
2021-09-28 16:48     ` Hao Xu
2021-09-29 11:23       ` Pavel Begunkov
2021-09-27  6:17 ` [PATCH 3/8] io_uring: add a limited tw list for irq completion work Hao Xu
2021-09-28 11:29   ` Pavel Begunkov
2021-09-28 16:55     ` Hao Xu
2021-09-29 11:25       ` Pavel Begunkov
2021-09-29 11:38     ` Hao Xu
2021-09-30  9:02       ` Pavel Begunkov
2021-09-30  3:21     ` Hao Xu
2021-09-27  6:17 ` [PATCH 4/8] io_uring: add helper for task work execution code Hao Xu
2021-09-27  6:17 ` [PATCH 5/8] io_uring: split io_req_complete_post() and add a helper Hao Xu
2021-09-27  6:17 ` [PATCH 6/8] io_uring: move up io_put_kbuf() and io_put_rw_kbuf() Hao Xu
2021-09-27  6:17 ` [PATCH 7/8] io_uring: add tw_ctx for io_uring_task Hao Xu
2021-09-27  6:17 ` [PATCH 8/8] io_uring: batch completion in prior_task_list Hao Xu
2021-09-27  6:21 ` [PATCH 0/6] task_work optimization Hao Xu
  -- strict thread matches above, loose matches on Subject: below --
2021-09-27 10:51 [PATCH v2 0/8] " Hao Xu
2021-09-27 10:51 ` [PATCH 1/8] io-wq: code clean for io_wq_add_work_after() Hao Xu

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