On 5/7/18 8:03 AM, Paolo Valente wrote: > Hi Jens, Christoph, all, > Mike Galbraith has been experiencing hangs, on blk_mq_get_tag, only > with bfq [1]. Symptoms seem to clearly point to a problem in I/O-tag > handling, triggered by bfq because it limits the number of tags for > async and sync write requests (in bfq_limit_depth). > > Fortunately, I just happened to find a way to apparently confirm it. > With the following one-liner for block/bfq-iosched.c: > > @@ -554,8 +554,7 @@ static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data) > if (unlikely(bfqd->sb_shift != bt->sb.shift)) > bfq_update_depths(bfqd, bt); > > - data->shallow_depth = > - bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)]; > + data->shallow_depth = 1; > > bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u", > __func__, bfqd->wr_busy_queues, op_is_sync(op), > > Mike's machine now crashes soon and systematically, while nothing bad > happens on my machines, even with heavy workloads (apart from an > expected throughput drop). > > This change simply reduces to 1 the maximum possible value for the sum > of the number of async requests and of sync write requests. > > This email is basically a request for help to knowledgeable people. To > start, here are my first doubts/questions: > 1) Just to be certain, I guess it is not normal that blk-mq hangs if > async requests and sync write requests can be at most one, right? > 2) Do you have any hint to where I could look for, to chase this bug? > Of course, the bug may be in bfq, i.e, it may be a somehow unrelated > bfq bug that causes this hang in blk-mq, indirectly. But it is hard > for me to understand how. CC Omar, since he implemented the shallow part. But we'll need some traces to show where we are hung, probably also the value of the /sys/debug/kernel/block/<dev>/ directory. For the crash mentioned, a trace as well. Otherwise we'll be wasting a lot of time on this. Is there a reproducer? -- Jens Axboe
