Messages in this thread | | | From | Paolo Valente <> | Subject | bug in tag handling in blk-mq? | Date | Mon, 7 May 2018 16:03:34 +0200 |
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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.
Looking forward to some help.
Thanks, Paolo
[1] https://www.spinics.net/lists/stable/msg215036.html
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