+static int blk_mq_hctx_notify_prepare(unsigned int cpu, struct hlist_node *node)
+{
+ struct blk_mq_hw_ctx *hctx;
+ struct blk_mq_tags *tags;
+
+ hctx = hlist_entry_safe(node, struct blk_mq_hw_ctx, cpuhp_dead);
+ tags = hctx->tags;
+
+ if (tags)
+ clear_bit(BLK_MQ_TAGS_DRAINED, &tags->flags);
+
Hi Ming,
Thanks for the effort here.
I would like to make an assertion on a related topic, which I hope you can
comment on:
For this drain mechanism to work, the blk_mq_hw_ctx’s (and related cpu
masks) for a request queue are required to match the hw queues used in the
LLDD (if using managed interrupts).
In others words, a SCSI LLDD needs to expose all hw queues for this to work.
The reason I say this is because if the LLDD does not expose the hw queues
and manages them internally - as some SCSI LLDDs do - yet uses managed
interrupts to spread the hw queue MSI vectors across all CPUs, then we still
only have a single blk_mq_hw_ctx per rq with a cpumask covering all cpus,
which is not what we would want.
Hi Ming,
Good catch!
This drain mechanism won't address the issue for these SCSI LLDDs in which:
1) blk_mq_hw_ctx serves as submission hw queue
2) one private reply queue serves as completion queue, for which one
MSI vector with cpumask is setup via pci_alloc_irq_vectors_affinity(PCI_IRQ_AFFINITY).
What we should only drain is the completion queue if all its mapped
CPUs are offline.
Looks you suggest to expose all completion(reply) queues as 'struct blk_mq_hw_ctx',
which may involve in another more hard problem: how to split the single
hostwide tags into each reply queue.
Yes, and this is what I expecting to hear Re. hostwide tags.
I'd rather not work towards that
direction because:
1) it is very hard to partition global resources into several parts,
especially it is hard to make every part happy.
2) sbitmap is smart/efficient enough for this global allocation
3) no obvious improvement is obtained from the resource partition, according
to previous experiment result done by Kashyap.
I'd like to also do the test.
However I would need to forward port the patchset, which no longer
cleanly applies (I was referring to this
https://lore.kernel.org/linux-block/20180205152035.15016-1-ming.lei@xxxxxxxxxx/).
Any help with that would be appreciated.
I think we could implement the drain mechanism in the following way:
1) if 'struct blk_mq_hw_ctx' serves as completion queue, use the
approach in the patch
Maybe the gain of exposing multiple queues+managed interrupts outweighs
the loss in the LLDD of having to generate this unique tag with sbitmap;
I know that we did not use sbitmap ever in the LLDD for generating the
tag when testing previously. However I'm still not too hopeful.
2) otherwise:
- introduce one callbcack of .prep_queue_dead(hctx, down_cpu) to
'struct blk_mq_ops'
This would not be allowed to block, right?
- call .prep_queue_dead from blk_mq_hctx_notify_dead()
3) inside .prep_queue_dead():
- the driver checks if all mapped CPU on the completion queue is offline
- if yes, wait for in-flight requests originated from all CPUs mapped to
this completion queue, and it can be implemented as one block layer API
That could work. However I think that someone may ask why the LLDD just
doesn't register for the CPU hotplug event itself (which I would really
rather avoid), instead of being relayed the info from the block layer.
Any comments on the above approach?
Thanks,
Ming
Cheers,
John
.
