On 13/11/2019 15:38, Hannes Reinecke wrote:

quoted

quoted

quoted

-        if (clear_ctx_on_error)
-            data->ctx = NULL;
-        blk_queue_exit(q);
-        return NULL;
+    if (data->hctx->shared_tags) {
+        shared_tag = blk_mq_get_shared_tag(data);
+        if (shared_tag == BLK_MQ_TAG_FAIL)
+            goto err_shared_tag;
       }
   -    rq = blk_mq_rq_ctx_init(data, tag, data->cmd_flags,
alloc_time_ns);
+    tag = blk_mq_get_tag(data);
+    if (tag == BLK_MQ_TAG_FAIL)
+        goto err_tag;
+
+    rq = blk_mq_rq_ctx_init(data, tag, shared_tag, data->cmd_flags,
alloc_time_ns);
       if (!op_is_flush(data->cmd_flags)) {
           rq->elv.icq = NULL;
           if (e && e->type->ops.prepare_request) {

Hi Hannes,

quoted

Why do you need to keep a parallel tag accounting between 'normal' and
'shared' tags here?
Isn't is sufficient to get a shared tag only, and us that in lieo of the
'real' one?

In theory, yes. Just the 'shared' tag should be adequate.

A problem I see with this approach is that we lose the identity of which
tags are allocated for each hctx. As an example for this, consider
blk_mq_queue_tag_busy_iter(), which iterates the bits for each hctx.
Now, if you're just using shared tags only, that wouldn't work.

Consider blk_mq_can_queue() as another example - this tells us if a hctx
has any bits unset, while with only using shared tags it would tell if
any bits unset over all queues, and this change in semantics could break
things. At a glance, function __blk_mq_tag_idle() looks problematic also.

And this is where it becomes messy to implement.

Hi Hannes,

Oh, my. Indeed, that's correct.

The tags could be kept in sync like this:

shared_tag = blk_mq_get_tag(shared_tagset);
if (shared_tag != -1)
	sbitmap_set(hctx->tags, shared_tag);

But that's obviously not ideal.

But then we don't really care _which_ shared tag is assigned; so
wouldn't be we better off by just having an atomic counter here?
Cache locality will be blown anyway ...

The atomic counter would solve the issuing more than Scsi_host.can_queue 
to the LLDD, but we still need a unique tag, which is what the shared 
tag is.

Thanks,
John