On Fri, Jun 30, 2017 at 2:42 AM, Jens Axboe <axboe@xxxxxxxxx> wrote: > On 06/29/2017 10:00 AM, Jens Axboe wrote: >> On 06/29/2017 09:58 AM, Jens Axboe wrote: >>> On 06/29/2017 02:40 AM, Ming Lei wrote: >>>> On Thu, Jun 29, 2017 at 5:49 AM, Jens Axboe <axboe@xxxxxxxxx> wrote: >>>>> On 06/28/2017 03:12 PM, Brian King wrote: >>>>>> This patch converts the in_flight counter in struct hd_struct from a >>>>>> pair of atomics to a pair of percpu counters. This eliminates a couple >>>>>> of atomics from the hot path. When running this on a Power system, to >>>>>> a single null_blk device with 80 submission queues, irq mode 0, with >>>>>> 80 fio jobs, I saw IOPs go from 1.5M IO/s to 11.4 IO/s. >>>>> >>>>> This has been done before, but I've never really liked it. The reason is >>>>> that it means that reading the part stat inflight count now has to >>>>> iterate over every possible CPU. Did you use partitions in your testing? >>>>> How many CPUs were configured? When I last tested this a few years ago >>>>> on even a quad core nehalem (which is notoriously shitty for cross-node >>>>> latencies), it was a net loss. >>>> >>>> One year ago, I saw null_blk's IOPS can be decreased to 10% >>>> of non-RQF_IO_STAT on a dual socket ARM64(each CPU has >>>> 96 cores, and dual numa nodes) too, the performance can be >>>> recovered basically if per numa-node counter is introduced and >>>> used in this case, but the patch was never posted out. >>>> If anyone is interested in that, I can rebase the patch on current >>>> block tree and post out. I guess the performance issue might be >>>> related with system cache coherency implementation more or less. >>>> This issue on ARM64 can be observed with the following userspace >>>> atomic counting test too: >>>> >>>> http://kernel.ubuntu.com/~ming/test/cache/ >>> >>> How well did the per-node thing work? Doesn't seem to me like it would >>> go far enough. And per CPU is too much. One potential improvement would >>> be to change the part_stat_read() to just loop online CPUs, instead of >>> all possible CPUs. When CPUs go on/offline, use that as the slow path to >>> ensure the stats are sane. Often there's a huge difference between >>> NR_CPUS configured and what the system has. As Brian states, RH ships >>> with 2048, while I doubt a lot of customers actually run that... >>> >>> Outside of coming up with a more clever data structure that is fully >>> CPU topology aware, one thing that could work is just having X cache >>> line separated read/write inflight counters per node, where X is some >>> suitable value (like 4). That prevents us from having cross node >>> traffic, and it also keeps the cross cpu traffic fairly low. That should >>> provide a nice balance between cost of incrementing the inflight >>> counting, and the cost of looping for reading it. >>> >>> And that brings me to the next part... >>> >>>>> I do agree that we should do something about it, and it's one of those >>>>> items I've highlighted in talks about blk-mq on pending issues to fix >>>>> up. It's just not great as it currently stands, but I don't think per >>>>> CPU counters is the right way to fix it, at least not for the inflight >>>>> counter. >>>> >>>> Yeah, it won't be a issue for non-mq path, and for blk-mq path, maybe >>>> we can use some blk-mq knowledge(tagset?) to figure out the >>>> 'in_flight' counter. I thought about it before, but never got a >>>> perfect solution, and looks it is a bit hard, :-) >>> >>> The tags are already a bit spread out, so it's worth a shot. That would >>> remove the need to do anything in the inc/dec path, as the tags already >>> do that. The inlight count could be easily retrieved with >>> sbitmap_weight(). The only issue here is that we need separate read and >>> write counters, and the weight would obviously only get us the total >>> count. But we can have a slower path for that, just iterate the tags and >>> count them. The fast path only cares about total count. >>> >>> Let me try that out real quick. >> >> Well, that only works for whole disk stats, of course... There's no way >> around iterating the tags and checking for this to truly work. > > Totally untested proof of concept for using the tags for this. I based > this on top of Brian's patch, so it includes his patch plus the > _double() stuff I did which is no longer really needed. > > > diff --git a/block/bio.c b/block/bio.c > index 9cf98b29588a..ec99d9ba0f33 100644 > --- a/block/bio.c > +++ b/block/bio.c > @@ -1737,7 +1737,7 @@ void generic_start_io_acct(int rw, unsigned long sectors, > part_round_stats(cpu, part); > part_stat_inc(cpu, part, ios[rw]); > part_stat_add(cpu, part, sectors[rw], sectors); > - part_inc_in_flight(part, rw); > + part_inc_in_flight(cpu, part, rw); > > part_stat_unlock(); > } > @@ -1751,7 +1751,7 @@ void generic_end_io_acct(int rw, struct hd_struct *part, > > part_stat_add(cpu, part, ticks[rw], duration); > part_round_stats(cpu, part); > - part_dec_in_flight(part, rw); > + part_dec_in_flight(cpu, part, rw); > > part_stat_unlock(); > } > diff --git a/block/blk-core.c b/block/blk-core.c > index af393d5a9680..6ab2efbe940b 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -2434,8 +2434,13 @@ void blk_account_io_done(struct request *req) > > part_stat_inc(cpu, part, ios[rw]); > part_stat_add(cpu, part, ticks[rw], duration); > - part_round_stats(cpu, part); > - part_dec_in_flight(part, rw); > + > + if (req->q->mq_ops) > + part_round_stats_mq(req->q, cpu, part); > + else { > + part_round_stats(cpu, part); > + part_dec_in_flight(cpu, part, rw); > + } > > hd_struct_put(part); > part_stat_unlock(); > @@ -2492,8 +2497,12 @@ void blk_account_io_start(struct request *rq, bool new_io) > part = &rq->rq_disk->part0; > hd_struct_get(part); > } > - part_round_stats(cpu, part); > - part_inc_in_flight(part, rw); > + if (rq->q->mq_ops) > + part_round_stats_mq(rq->q, cpu, part); > + else { > + part_round_stats(cpu, part); > + part_inc_in_flight(cpu, part, rw); > + } > rq->part = part; > } > > diff --git a/block/blk-merge.c b/block/blk-merge.c > index 99038830fb42..3b5eb2d4b964 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -634,7 +634,7 @@ static void blk_account_io_merge(struct request *req) > part = req->part; > > part_round_stats(cpu, part); > - part_dec_in_flight(part, rq_data_dir(req)); > + part_dec_in_flight(cpu, part, rq_data_dir(req)); > > hd_struct_put(part); > part_stat_unlock(); > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c > index d0be72ccb091..a7b897740c47 100644 > --- a/block/blk-mq-tag.c > +++ b/block/blk-mq-tag.c > @@ -214,7 +214,7 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data) > bitnr += tags->nr_reserved_tags; > rq = tags->rqs[bitnr]; > > - if (rq->q == hctx->queue) > + if (rq && rq->q == hctx->queue) > iter_data->fn(hctx, rq, iter_data->data, reserved); > return true; > } > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 05dfa3f270ae..cad4d2c26285 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -43,6 +43,58 @@ static LIST_HEAD(all_q_list); > static void blk_mq_poll_stats_start(struct request_queue *q); > static void blk_mq_poll_stats_fn(struct blk_stat_callback *cb); > > +struct mq_inflight { > + struct hd_struct *part; > + unsigned int inflight; > +}; > + > +static void blk_mq_check_inflight(struct blk_mq_hw_ctx *hctx, > + struct request *rq, void *priv, > + bool reserved) > +{ > + struct mq_inflight *mi = priv; > + > + if (rq->part == mi->part && > + test_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) > + mi->inflight++; > +} > + > +unsigned long part_in_flight_mq(struct request_queue *q, > + struct hd_struct *part) > +{ > + struct mq_inflight mi = { .part = part, .inflight = 0 }; > + > + blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi); > + return mi.inflight; > +} Compared with the totally percpu approach, this way might help 1:M or N:M mapping, but won't help 1:1 map(NVMe), when hctx is mapped to each CPU(especially there are huge hw queues on a big system), :-( Thanks, Ming Lei -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
