On Thu, May 24, 2018 at 1:48 AM, Omar Sandoval <osandov@xxxxxxxxxxx> wrote: > 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? What matters is that this patch wakes up the previous sbq, let's see if from another view: 1) still 2 hw queues, nr_requests are 2, and wake_batch is one 2) there are 3 waiters on hw queue 0 3) two in-flight requests in hw queue 0 are completed, and only two waiters of 3 are waken up because of wake_batch, but both the two waiters can be scheduled to another CPU and cause to switch to hw queue 1 4) then the 3rd waiter will wait for ever, since no in-flight request is in hw queue 0 any more. 5) this patch fixes it by the fake wakeup when waiter is scheduled to another hw queue The issue can be understood a bit easier if we just forget sbq_wait_state and focus on sbq, :-) Thanks, Ming Lei
