> Il giorno 07 mag 2018, alle ore 18:39, Jens Axboe <axboe@xxxxxxxxx> ha scritto: > > 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? > Ok Mike, I guess it's your turn now, for at least a stack trace. Thanks, Paolo > -- > Jens Axboe
