linux-kernel - Re: [PATCH 1/2] io_uring: trigger timeout after any sqe->off CQEs

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Date:   Mon, 20 Apr 2020 23:15:30 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] io_uring: trigger timeout after any sqe->off CQEs

On 20/04/2020 23:12, Pavel Begunkov wrote:
> On 20/04/2020 22:40, Jens Axboe wrote:
>> On 4/18/20 11:20 AM, Pavel Begunkov wrote:
>>> +static void __io_flush_timeouts(struct io_ring_ctx *ctx)
>>> +{
>>> +	u32 end, start;
>>> +
>>> +	start = end = ctx->cached_cq_tail;
>>> +	do {
>>> +		struct io_kiocb *req = list_first_entry(&ctx->timeout_list,
>>> +							struct io_kiocb, list);
>>> +
>>> +		if (req->flags & REQ_F_TIMEOUT_NOSEQ)
>>> +			break;
>>> +		/*
>>> +		 * multiple timeouts may have the same target,
>>> +		 * check that @req is in [first_tail, cur_tail]
>>> +		 */
>>> +		if (!io_check_in_range(req->timeout.target_cq, start, end))
>>> +			break;
>>> +
>>> +		list_del_init(&req->list);
>>> +		io_kill_timeout(req);
>>> +		end = ctx->cached_cq_tail;
>>> +	} while (!list_empty(&ctx->timeout_list));
>>> +}
>>> +
>>>  static void io_commit_cqring(struct io_ring_ctx *ctx)
>>>  {
>>>  	struct io_kiocb *req;
>>>  
>>> -	while ((req = io_get_timeout_req(ctx)) != NULL)
>>> -		io_kill_timeout(req);
>>> +	if (!list_empty(&ctx->timeout_list))
>>> +		__io_flush_timeouts(ctx);
>>>  
>>>  	__io_commit_cqring(ctx);
>>>  
>>
>> Any chance we can do this without having to iterate timeouts on the
>> completion path?
>>
> 
> If you mean the one in __io_flush_timeouts(), then no, unless we forbid timeouts
> with identical target sequences + some extra constraints. The loop there is not
> new, it iterates only over timeouts, that need to be completed, and removes
> them. That's amortised O(1).

We can think about adding unlock/lock, if that's what you are thinking about.


> On the other hand, there was a loop in io_timeout_fn() doing in total O(n^2),
> and it was killed by this patch.

-- 
Pavel Begunkov