On Tue, 4 Nov 2008, Tejun Heo wrote: > Hello, Alan Stern! :-) > > Alan Stern wrote: > > Even a "peek and fetch" interface might not be best, at least as far as > > timer issues are concerned. Ideally, the timer shouldn't be started > > until the SCSI midlayer knows that the request has successfully been > > sent to the lower-level driver. > > > > Therefore the best approach would be to EXPORT blk_add_timer(). It > > should be called at the end of scsi_dispatch_cmd(), when the return > > value from the queuecommand method is known to be 0. > > > > With something like this, Mike's fix to end_that_request_last() > > wouldn't be needed, since blkdev_dequeue_request() wouldn't > > automatically start the timer. It seems silly to start the timer when > > you know you're just going to stop it immediately afterwards. > > Block layer currently doesn't know when a request is actually being > issued. For timeout, blk_add_timer() can be exported but I think that > only aggravate the already highly fragmented block layer interface > (different users use it differently to the point of scary chaos). For > minor example, block tracing considers elv_next_request() as the command > issue point which isn't quite true for SCSI and many other drivers. For > that too, we can export the tracing interface but I don't think that's > the right direction. More stuff are scheduled to be moved to block > layer and exporting more and more implementation details to block layer > users will have hard time scaling. > > I'm trying to convert all drivers to use the same command issue model - > elv_next_request() -> blkdev_dequeue_request() on actual issue -> > blk_end_request(). I have first draft of the conversion patchset but > it's gonna take me a few more days to review and test what I can as > several drivers (mostly legacy ones) are a bit tricky. > > For the time being, SCSI layer is the only block layer timeout user and > completion w/o dequeuing is only for error cases in SCSI, so the > inefficiency there shouldn't matter too much. In fact, I have changed my mind. Starting the timer after the command has been sent to the low-level driver would mean that the command might finish before the timer was started! So never mind. I did confirm at least that your patch together with Mike's fixed the problem I encountered last week. I have a couple of small fixes for the block timer routines. They'll get posted separately later on. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
