On 04/06/2017 08:27 AM, Arun Easi wrote:
> Hi Hannes,
>
> Thanks for taking a crack at the issue. My comments below..
>
> On Tue, 4 Apr 2017, 5:07am, Hannes Reinecke wrote:
>
>> Most legacy HBAs have a tagset per HBA, not per queue. To map
>> these devices onto block-mq this patch implements a new tagset
>> flag BLK_MQ_F_GLOBAL_TAGS, which will cause the tag allocator
>> to use just one tagset for all hardware queues.
>>
>> Signed-off-by: Hannes Reinecke <h...@suse.com>
>> ---
>> block/blk-mq-tag.c | 12 ++++++++----
>> block/blk-mq.c | 10 ++++++++--
>> include/linux/blk-mq.h | 1 +
>> 3 files changed, 17 insertions(+), 6 deletions(-)
>>
>> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
>> index e48bc2c..a14e76c 100644
>> --- a/block/blk-mq-tag.c
>> +++ b/block/blk-mq-tag.c
>> @@ -276,9 +276,11 @@ static void blk_mq_all_tag_busy_iter(struct blk_mq_tags
>> *tags,
>> void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
>> busy_tag_iter_fn *fn, void *priv)
>> {
>> - int i;
>> + int i, lim = tagset->nr_hw_queues;
>>
>> - for (i = 0; i < tagset->nr_hw_queues; i++) {
>> + if (tagset->flags & BLK_MQ_F_GLOBAL_TAGS)
>> + lim = 1;
>> + for (i = 0; i < lim; i++) {
>> if (tagset->tags && tagset->tags[i])
>> blk_mq_all_tag_busy_iter(tagset->tags[i], fn, priv);
>> }
>> @@ -287,12 +289,14 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set
>> *tagset,
>>
>> int blk_mq_reinit_tagset(struct blk_mq_tag_set *set)
>> {
>> - int i, j, ret = 0;
>> + int i, j, ret = 0, lim = set->nr_hw_queues;
>>
>> if (!set->ops->reinit_request)
>> goto out;
>>
>> - for (i = 0; i < set->nr_hw_queues; i++) {
>> + if (set->flags & BLK_MQ_F_GLOBAL_TAGS)
>> + lim = 1;
>> + for (i = 0; i < lim; i++) {
>> struct blk_mq_tags *tags = set->tags[i];
>>
>> for (j = 0; j < tags->nr_tags; j++) {
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 159187a..db96ed0 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -2061,6 +2061,10 @@ static bool __blk_mq_alloc_rq_map(struct
>> blk_mq_tag_set *set, int hctx_idx)
>> {
>> int ret = 0;
>>
>> + if ((set->flags & BLK_MQ_F_GLOBAL_TAGS) && hctx_idx != 0) {
>> + set->tags[hctx_idx] = set->tags[0];
>> + return true;
>> + }
>
> So, this effectively make all request allocations to the same NUMA node
> locality of the hctx_idx 0, correct? Is the performance hit you were
> talking about in the cover letter?
>
Yes. It does make the request allocations local to NUMA node 0, but then
this will only affect LLDDs which are actually _using_ NUMA locality
when allocating the request nodes.
However, SCSI doesn't set a NUMA node locality to begin with, so this
doesn't affect us.
No, what I meant is this:
the 'sbitmap' allocator already splits up the bitmap into several words,
which then should provide a better NUMA locality per map.
When we're using a shared global map it's unclear whether the individual
words of the sbitmap can and will be moved to the various NUMA nodes, or
whether we suffer from non-locality.
My tests so far have been inconclusive; but then I'm not happy with the
testcase anyway (using null_blk I only get 250k/250k r/w IOPs, which I
found rather disappointing).
> Do you have any other alternatives in mind? Dynamic growing/shrinking
> tags/request-pool in hctx with a fixed base as start?
>
> One alternative that comes to my mind is to move the divvy up logic to
> SCSI (instead of LLD doing it), IOW:
>
> 1. Have SCSI set tag_set.queue_depth to can_queue/nr_hw_queues
> 2. Have blk_mq_unique_tag() (or a new i/f) returning "hwq * nr_hw_queue +
> rq->tag"
>
> That would make the tags linear in the can_queue space, but could result
> in poor use of LLD resource if a given hctx has used up all it's tags.
>
Exactly. This is the method I used for implementing mq support for lpfc
and mpt3sas; however the complaint there indeed was that we might be
running into a tag starvation scenario with a large number of LUNs and
single-threaded I/O submission.
> On a related note, would not the current use of can_queue in SCSI lead to
> poor resource utilization in MQ cases? Like, block layer allocating
> nr_hw_queues * tags+request+driver_data.etc * can_queue, but SCSI limiting
> the number of requests to can_queue.
>
Yes, indeed. That's another problem which we should be looking at.
However, it's only ever relevant if we indeed implement some divvying
logic; if we move to the shared tags approach it should work as designed.
> BTW, if you would like me to try out this patch on my setup, please let me
> know.
>
Oh, yes. Please do.
Cheers,
Hannes
--
Dr. Hannes Reinecke zSeries & Storage
h...@suse.com +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
