Re: [PATCH v2] io-wq: handle hashed writes in chains

[Pavel Begunkov <asml.silence@gmail.com>]

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On 23/03/2020 04:37, Jens Axboe wrote:
> On 3/22/20 2:25 PM, Pavel Begunkov wrote:
>> On 22/03/2020 22:51, Jens Axboe wrote:
>>> commit f1d96a8fcbbbb22d4fbc1d69eaaa678bbb0ff6e2
>>> Author: Pavel Begunkov <asml.silence@gmail.com>
>>> Date:   Fri Mar 13 22:29:14 2020 +0300
>>>
>>>     io_uring: NULL-deref for IOSQE_{ASYNC,DRAIN}
>>>
>>> which is what I ran into as well last week...
>>
>> I picked it before testing
>>
>>> The extra memory isn't a bit deal, it's very minor. My main concern
>>> would be fairness, since we'd then be grabbing non-contig hashed chunks,
>>> before we did not. May not be a concern as long as we ensure the
>>> non-hasned (and differently hashed) work can proceed in parallel. For my
>>> end, I deliberately added:
>>
>> Don't think it's really a problem, all ordering/scheduling is up to
>> users (i.e.  io_uring), and it can't infinitely postpone a work,
>> because it's processing spliced requests without taking more, even if
>> new ones hash to the same bit.
> 
> I don't disagree with you, just wanted to bring it up!

Sure, there is a lot to think about. E.g. I don't like this reenqueueing,
and if all other thread have enough work to do, then it can avoided, but don't
want to over-complicate.


> 
>>> +	/* already have hashed work, let new worker get this */
>>> +	if (ret) {
>>> +		struct io_wqe_acct *acct;
>>> +
>>> +		/* get new worker for unhashed, if none now */
>>> +		acct = io_work_get_acct(wqe, work);
>>> +		if (!atomic_read(&acct->nr_running))
>>> +			io_wqe_wake_worker(wqe, acct);
>>> +		break;
>>> +	}
>>>
>>> to try and improve that.
>>
>> Is there performance problems with your patch without this chunk? I
>> may see another problem with yours, I need to think it through.
> 
> No, and in fact it probably should be a separate thing, but I kind of
> like your approach so not moving forward with mine. I do think it's
> worth looking into separately, as there's no reason why we can't wake a
> non-hashed worker if we're just doing hashed work from the existing
> thread. If that thread is just doing copies and not blocking, the
> unhashed (or next hashed) work is just sitting idle while it could be
> running instead.

Then, I'll clean the diff, hopefully soon. Could I steal parts of your patch
description?

> 
> Hence I added that hunk, to kick a new worker to proceed in parallel.

It seems, I need to take a closer look at this accounting in general.


-- 
Pavel Begunkov


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