On Tue, May 22, 2018 at 02:20:37PM -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. > > Can you explain what the actual bug is? The commit message doesn't > really say, it just explains what the code change does. What wake up > miss?
For example: 1) one request queue has 2 queues, and queue depth is low, so wake_batch is set 1 or very small. 2) There are 2 allocations which are blocked on hw queue 0, now the last request mapped to hw queue 0 is completed, then sbq_wake_up() wakes only one of 2 blockers. 3) the waken up allocation process is migrated to another CPU, and its hw queue becomes mapped to hw queue 1, and this allocation is switched to hw queue 1 4) so there isn't any chance for another blocked allocation to move one, since all request in hw queue 0 has been completed. That is why I call it wake up miss. BTW, this issue can be reproduced reliably by the block/020 I posted yesterday: https://marc.info/?l=linux-block&m=152698758418536&w=2 Thanks, Ming
