Re: [PATCH RFC 10/14] block, bfq: add Early Queue Merge (EQM)

[Paolo Valente]

> Il giorno 15 mar 2017, alle ore 21:00, Jens Axboe <axboe@xxxxxxxxx> ha scritto:
> 
> On 03/15/2017 10:59 AM, Paolo Valente wrote:
>> 
>>> Il giorno 15 mar 2017, alle ore 17:30, Jens Axboe <axboe@xxxxxxxxx> ha scritto:
>>> 
>>> On 03/15/2017 09:47 AM, Jens Axboe wrote:
>>>> I think you understood me correctly. Currently I think the putting of
>>>> the io context is somewhat of a mess. You have seemingly random places
>>>> where you have to use special unlock functions, to ensure that you
>>>> notice that some caller deeper down has set ->ioc_to_put. I took a quick
>>>> look at it, and by far most of the cases can return an io_context to
>>>> free quite easily. You can mark these functions __must_check to ensure
>>>> that we don't drop an io_context, inadvertently. That's already a win
>>>> over the random ->ioc_to_put store. And you can then get rid of
>>>> bfq_unlock_put_ioc and it's irq variant as well.
>>>> 
>>>> The places where you are already returning a value, like off dispatch
>>>> for instance, you can just pass in a pointer to an io_context pointer.
>>>> 
>>>> If you get this right, it'll be a lot less fragile and hacky than your
>>>> current approach.
>>> 
>>> Even just looking a little closer, you also find cases where you
>>> potentially twice store ->ioc_to_put. That kind of mixup can't happen if
>>> you return it properly.
>>> 
>>> In __bfq_dispatch_request(), for instance. You call bfq_select_queue(),
>>> and that in turn calls bfq_bfqq_expire(), which calls
>>> __bfq_bfqq_expire() which can set ->ioc_to_put. But later on,
>>> __bfq_dispatch_request() calls bfq_dispatch_rq_from_bfqq(), which in
>>> turn calls bfq_bfqq_expire() that can also set ->ioc_to_put. There's no
>>> "magic" bfq_unlock_and_put_ioc() in-between those. Maybe the former call
>>> never sets ->ioc_to_put if it returns with bfqq == NULL? Hard to tell.
>>> 
>>> Or __bfq_insert_request(), it calls bfq_add_request(), which may set
>>> ->ioc_to_put through bfq_bfqq_handle_idle_busy_switch() ->
>>> bfq_bfqq_expire(). And then from calling bfq_rq_enqueued() ->
>>> bfq_bfqq_expire().
>>> 
>> 
>> I have checked that.  Basically, since a queue can't be expired twice,
>> then it should never happen that ioc_to_put is set twice before being
>> used.  Yet, I do agree that using a shared field and exploiting
>> collateral effects makes code very complex and fragile (maybe even
>> buggy if my speculative check is wrong).  Just, it has been the best
>> solution I found, to avoid deferred work as you asked.  In fact, I
>> still find quite heavy the alternative of passing a pointer to an ioc
>> forth and back across seven or eight nested functions.
> 
> It's not heavy at all, I went through all of it this morning.

Yes, sorry. I meant heavy in terms of code complexity.

> It's not
> super pretty either, since you end up passing back an io_context which
> is seemingly unrelated to what the functions otherwise do.

Exactly.

> But that's
> mostly a reflection of the implementation, not that it's a bad way to go
> about this in general. The worst bits are the places where you want to
> add a
> 
> 	WARN_ON(ret != NULL);
> 
> between two calls that potentially both drop the ioc. In terms of
> overhead, it's not heavy. Punting to a workqueue would be orders of
> magnitude more expensive.
> 
>>> There might be more, but I think the above is plenty of evidence that
>>> the current ->ioc_to_put solution is a bad hack, fragile, and already
>>> has bugs.
>>> 
>>> How often do you expect this putting of the io_context to happen?
>> 
>> Unfortunately often, as it must be done also every time the in-service
>> queue is reset.  But, in this respect, are we sure that we do need to
>> grab a reference to the ioc when we set a queue in service (as done in
>> cfq, and copied into bfq)?  I mean, we have the hook exit_ioc for
>> controlling the disappearing of an ioc.  Am I missing something here
>> too?
> 
> No, in fact that'd be perfectly fine. It's easier for CFQ to just retain
> the reference so we know it's not going away, but for your case, it
> might in fact make more sense to simply be able to de-service a queue if
> the process exits. And if you do that, we can drop all this passing back
> of ioc (or ->ioc_to_put) craziness, without having to punt to a
> workqueue either.
> 

Done, and ... well, it seems to work :)

I'm striving to have a new patch series ready before Monday, but I'm
not confident I'll make it.

Thanks,
Paolo

> This will be more efficient too, since it'll be a much more rare
> occurence.
> 
> -- 
> Jens Axboe