On 12/21/17 2:34 PM, Keith Busch wrote: > On Thu, Dec 21, 2017 at 02:00:04PM -0700, Jens Axboe wrote: >> On 12/21/17 1:56 PM, Scott Bauer wrote: >>> On 12/21/2017 01:46 PM, Keith Busch wrote: >>>> @@ -181,7 +181,10 @@ static void blkdev_bio_end_io_simple(struct bio *bio) >>>> struct task_struct *waiter = bio->bi_private; >>>> >>>> WRITE_ONCE(bio->bi_private, NULL); >>>> - wake_up_process(waiter); >>>> + if (current != waiter) >>>> + wake_up_process(waiter); >>>> + else >>>> + __set_current_state(TASK_RUNNING); >>> >>> Do we actually need to set this to TASK_RUNNING? If we get here we're already running, right? >>> >>> Everywhere I see uses of __set_current_state(TASK_RUNNING) it's after we've done a set_current_state(TASK_INTERRUPTIBLE). >> >> We'd only be TASK_RUNNING if the IRQ got to it first. And that's something that >> should be removed as well - I suspect that'd be a much bigger win, getting rid >> of the IRQ trigger for polled IO, than most of the micro optimizations. For >> Keith's testing, looks like he reduced the cost by turning on coalescing, but >> it'd be cheaper (and better) to not have to rely on that. > > It would be nice, but the driver doesn't know a request's completion > is going to be a polled. That's trivially solvable though, since the information is available at submission time. > Even if it did, we don't have a spec defined > way to tell the controller not to send an interrupt with this command's > compeletion, which would be negated anyway if any interrupt driven IO > is mixed in the same queue. We could possibly create a special queue > with interrupts disabled for this purpose if we can pass the HIPRI hint > through the request. There's on way to do it per IO, right. But you can create a sq/cq pair without interrupts enabled. This would also allow you to scale better with multiple users of polling, a case where we currently don't perform as well spdk, for instance. -- Jens Axboe
