Re: [PATCH RFC 3/5] blk-mq: Facilitate a shared tags per tagset

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@@ -341,8 +341,11 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
      int i;

      for (i = 0; i < tagset->nr_hw_queues; i++) {
-        if (tagset->tags && tagset->tags[i])
+        if (tagset->tags && tagset->tags[i]) {
              blk_mq_all_tag_busy_iter(tagset->tags[i], fn, priv);

As I mentioned earlier, wouldn't this iterate over all tags for all hctx's, when we just want the tags for hctx[i]?

Thanks,
John

[Not trimming reply for future reference]

+            if (tagset->share_tags)
+                break;
+        }
      }
  }
  EXPORT_SYMBOL(blk_mq_tagset_busy_iter);

Since blk_mq_tagset_busy_iter() loops over all hardware queues all what is changed is the order in which requests are examined. I am not aware of any block driver that calls blk_mq_tagset_busy_iter() and that depends on the order of the requests passed to the callback function.


OK, fine.

So, to me, this approach also seems viable then.

I am however not so happy with how we use blk_mq_tag_set.tags[0] for the shared tags; I would like to use blk_mq_tag_set.shared_tags and make blk_mq_tag_set.tags[] point at blk_mq_tag_set.shared_tags or maybe not blk_mq_tag_set.tags[] at all. However maybe that change may be more intrusive.

And another more real concern is that we miss a check somewhere for rq->mq_hctx == hctx when examining the bits on the shared tags.

Another issue I notice is that using tags from just hctx0 may cause a breakage when associated with a different hw queue in the driver.

Specifically we may have blk_mq_alloc_rqs(hctx_idx = 0)->blk_mq_init_request()->nvme_init_request(), and we would set all iod->nvmeq = nvmeq0; since we may actually use this iod on another hw queue, we're breaking that interface.

Thanks,
John





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