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

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Date:   Wed, 22 Apr 2020 16:23:51 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, 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 4/22/20 4:20 PM, Pavel Begunkov wrote:
> On 20/04/2020 23:15, Pavel Begunkov wrote:
>> 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.
>>
> 
> Any thoughts on this?
> 
> I'll return fixing the last timeout bug I saw, but I'd prefer to know
> on top of what to do that.

I think it's fine, but also likely something that we should defer to
5.8. So if there are minor fixes to be done for 5.7, it should be
arranged as such.

-- 
Jens Axboe