On Wed, May 23, 2018 at 05:32:31PM +0800, Ming Lei wrote: > On Tue, May 22, 2018 at 09:59:17PM -0600, Jens Axboe wrote: > > On 5/19/18 1:44 AM, Ming Lei wrote: > > > When the allocation process is scheduled back and the mapped hw queue is > > > changed, do one extra wake up on orignal queue for compensating wake up > > > miss, so other allocations on the orignal queue won't be starved. > > > > > > This patch fixes one request allocation hang issue, which can be > > > triggered easily in case of very low nr_request. > > > > Trying to think of better ways we can fix this, but I don't see > > any right now. Getting rid of the wake_up_nr() kills us on tons > > of tasks waiting. > > I am not sure if I understand your point, but this issue isn't related > with wake_up_nr() actually, and it can be reproduced after reverting > 4e5dff41be7b5201c1c47c (blk-mq: improve heavily contended tag case). > > All tasks in current sbq_wait_state may be scheduled to other CPUs, and > there may still be tasks waiting for allocation from this sbitmap_queue, > and the root cause is about cross-queue allocation, as you said, > there are too many queues, :-) I don't follow. Your description of the problem was that we have two waiters and only wake up one, which doesn't in turn allocate and free a tag and wake up the second waiter. Changing it back to wake_up_nr() eliminates that problem. And if waking up everything doesn't fix it, how does your fix of waking up a few extra tasks fix it?
