> 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. > 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? Thanks, Paolo > If > it's not a very frequent occurence, maybe using a deferred workqueue to > put it IS the right solution. As it currently stands, the code doesn't > really work, and it's fragile. It can't be cleaned up without > refactoring, since the call paths are all extremely intermingled. > > -- > Jens Axboe >
