Hi John, On Tue, May 21, 2019 at 02:50:06PM +0100, John Garry wrote: > On 17/05/2019 10:14, Ming Lei wrote: > > Managed interrupts can not migrate affinity when their CPUs are offline. > > If the CPU is allowed to shutdown before they're returned, commands > > dispatched to managed queues won't be able to complete through their > > irq handlers. > > > > Wait in cpu hotplug handler until all inflight requests on the tags > > are completed or timeout. Wait once for each tags, so we can save time > > in case of shared tags. > > > > Based on the following patch from Keith, and use simple delay-spin > > instead. > > > > https://lore.kernel.org/linux-block/20190405215920.27085-1-keith.busch@xxxxxxxxx/ > > > > Cc: Christoph Hellwig <hch@xxxxxx> > > Cc: Bart Van Assche <bvanassche@xxxxxxx> > > Cc: Hannes Reinecke <hare@xxxxxxxx> > > Cc: John Garry <john.garry@xxxxxxxxxx> > > Cc: Keith Busch <keith.busch@xxxxxxxxx> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> > > --- > > block/blk-mq-tag.c | 2 +- > > block/blk-mq-tag.h | 5 ++++ > > block/blk-mq.c | 65 +++++++++++++++++++++++++++++++++++++++++++++- > > 3 files changed, 70 insertions(+), 2 deletions(-) > > > > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c > > index 7513c8eaabee..b24334f99c5d 100644 > > --- a/block/blk-mq-tag.c > > +++ b/block/blk-mq-tag.c > > @@ -332,7 +332,7 @@ static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt, > > * true to continue iterating tags, false to stop. > > * @priv: Will be passed as second argument to @fn. > > */ > > -static void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags, > > +void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags, > > busy_tag_iter_fn *fn, void *priv) > > { > > if (tags->nr_reserved_tags) > > diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h > > index 61deab0b5a5a..f8de50485b42 100644 > > --- a/block/blk-mq-tag.h > > +++ b/block/blk-mq-tag.h > > @@ -19,6 +19,9 @@ struct blk_mq_tags { > > struct request **rqs; > > struct request **static_rqs; > > struct list_head page_list; > > + > > + #define BLK_MQ_TAGS_DRAINED 0 > > + unsigned long flags; > > }; > > > > > > @@ -35,6 +38,8 @@ extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx, > > extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool); > > void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn, > > void *priv); > > +void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags, > > + busy_tag_iter_fn *fn, void *priv); > > > > static inline struct sbq_wait_state *bt_wait_ptr(struct sbitmap_queue *bt, > > struct blk_mq_hw_ctx *hctx) > > diff --git a/block/blk-mq.c b/block/blk-mq.c > > index 08a6248d8536..d1d1b1a9628f 100644 > > --- a/block/blk-mq.c > > +++ b/block/blk-mq.c > > @@ -2214,6 +2214,60 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags, > > return -ENOMEM; > > } > > > > +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. 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. 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 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 2) otherwise: - introduce one callbcack of .prep_queue_dead(hctx, down_cpu) to 'struct blk_mq_ops' - 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 Any comments on the above approach? Thanks, Ming
