On 06/29/2017 09:17 PM, Jens Axboe wrote: > On 06/29/2017 07:20 PM, Ming Lei wrote: >> 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), :-( > > Not disagreeing with that, without having some mechanism to only > loop queues that have pending requests. That would be similar to the > ctx_map for sw to hw queues. But I don't think that would be worthwhile > doing, I like your pnode approach better. However, I'm still not fully > convinced that one per node is enough to get the scalability we need. > > Would be great if Brian could re-test with your updated patch, so we > know how it works for him at least. I'll try running with both approaches today and see how they compare. Thanks!!! Brian -- Brian King Power Linux I/O IBM Linux Technology Center
