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. -- Jens Axboe
