On Thu, May 24, 2018 at 06:09:51AM +0800, Ming Lei wrote: > 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, :-) Okay, I see, although if I'm understanding correctly, it has everything to do with sbq_wait_state. If we only had one waitqueue, then wake_up() would always wake up all of the waiters, but because we have them spread out over multiple waitqueues, we have to call sbq_wake_up()/sbitmap_queue_wake_up() to do the wake up on the other waitqueue.
