On 11/19/2013 02:43 PM, Jens Axboe wrote: > On 11/19/2013 02:34 PM, Dave Chinner wrote: >> On Tue, Nov 19, 2013 at 02:20:42PM -0700, Jens Axboe wrote: >>> On Tue, Nov 19 2013, Jens Axboe wrote: >>>> On Tue, Nov 19 2013, Dave Chinner wrote: >>>>> Hi Jens, >>>>> >>>>> I was just running xfstests on a 3.13 kernel that has had the block >>>>> layer changed merged into it. generic/269 on XFS is hanging on a 2 >>>>> CPU VM using virtio,cache=none for the block devices under test, >>>>> with many (130+) threads stuck below submit_bio() like this: >>>>> >>>>> Call Trace: >>>>> [<ffffffff81adb1c9>] schedule+0x29/0x70 >>>>> [<ffffffff817833ee>] percpu_ida_alloc+0x16e/0x330 >>>>> [<ffffffff81759bef>] blk_mq_wait_for_tags+0x1f/0x40 >>>>> [<ffffffff81758bee>] blk_mq_alloc_request_pinned+0x4e/0xf0 >>>>> [<ffffffff8175931b>] blk_mq_make_request+0x3bb/0x4a0 >>>>> [<ffffffff8174d2b2>] generic_make_request+0xc2/0x110 >>>>> [<ffffffff8174e40c>] submit_bio+0x6c/0x120 >>>>> >>>>> reads and writes are hung, both data (direct and buffered) and >>>>> metadata. >>>>> >>>>> Some IOs are sitting in io_schedule, waiting for IO completion (both >>>>> buffered and direct IO, both reads and writes) so it looks like IO >>>>> completion has stalled in some manner, too. >>>> >>>> Can I get a recipe to reproduce this? I haven't had any luck so far. >>> >>> OK, I reproduced it. Looks weird, basically all 64 commands are in >>> flight, but haven't completed. So the next one that comes in just sits >>> there forever. I can't find any sysfs debug entries for virtio, would be >>> nice to inspect its queue as well... >> >> Does it have anything to do with the fact that the request queue >> depth is 128 entries and the tag pool only has 66 tags in it? i.e: >> >> /sys/block/vdb/queue/nr_requests >> 128 >> >> /sys/block/vdb/mq/0/tags >> nr_tags=66, reserved_tags=2, batch_move=16, max_cache=32 >> nr_free=0, nr_reserved=1 >> cpu00: nr_free=0 >> cpu01: nr_free=0 >> >> Seems to imply that if we queue up more than 66 IOs without >> dispatching them, we'll run out of tags. And without another IO >> coming through, the "none" scheduler that virtio uses will never >> get a trigger to push out the currently queued IO? > > No, the nr_requests isn't actually relevant in the blk-mq context, the > driver sets its own depth. For the above, it's 64 normal commands, and 2 > reserved. The reserved would be for a flush, for instance. If someone > attempts to queue more than the allocated number of requests, it'll stop > the blk-mq queue and kick things into gear on the virtio side. Then when > requests complete, we start the queue again. > > If you look at virtio_queue_rq(), that handles a single request. This > request is already tagged at this point. If we can't add it to the ring, > we simply stop the queue and kick off whatever pending we might have. We > return BLK_MQ_RQ_QUEUE_BUSY to blk-mq, which tells that to back off on > sending us more. When we get the virtblk_done() callback from virtio, we > end the requests on the blk-mq side and restart the queue.
I added some debug code to see if we had anything pending on the blk-mq side, and it's all empty. It really just looks like we are missing completions on the virtio side. Very odd. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
