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: Wed, 29 Sep 2021 12:23:07 +0100 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
On 9/29/21 8:36 AM, Hao Xu wrote:
> 在 2021/9/28 下午7:08, Pavel Begunkov 写道:
>> 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-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
> hmm, this is definitely not good, not sure why this is not optimised to
> cmpq $0, %rax (haven't touched assembly for a long time..)
Nothing strange, alias analysis, it can't infer that the pointers
don't point to overlapping memory, and so can do nothing but reload.
__restrict__ C keyword would've helped, but it's not used in
the kernel.
--
Pavel Begunkov
next prev parent reply other threads:[~2021-09-29 11:23 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
2021-09-29 7:36 ` Hao Xu
2021-09-29 11:23 ` Pavel Begunkov [this message]
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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