On 06/30/2017 10:17 PM, Jens Axboe wrote: > On 06/30/2017 08:08 AM, Jens Axboe wrote: >> On 06/30/2017 07:05 AM, Brian King wrote: >>> 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. >> >> Focus on Ming's, a variant of that is the most likely path forward, >> imho. It'd be great to do a quick run on mine as well, just to establish >> how it compares to mainline, though. > > Here's a cleaned up series: > > http://git.kernel.dk/cgit/linux-block/log/?h=mq-inflight > > (it's against mainline for now, I will update it to be against > for-4.13/block in a rebase). > > One optimization on top of this I want to do is to only iterate once, > even for a partition - pass in both parts, and increment two different > counts. If we collapse the two part time stamps, then that's doable, and > it means we only have to iterate once. > > Essentially this series makes the inc/dec a noop, since we don't have to > do anything. The reading is basically no worse than a cpu online > iteration, since we never have more queues than online CPUs. That's an > improvement over per-cpu for-each-possible loops. For a lot of cases, > it's much less, since we have fewer queues than CPUs. I'll need an hour > or two to hone this a bit more, but then it would be great if you can > retest. I'll send out an email when that's done, it'll be some time over > this weekend. Did the double-read with one iteration change, it was pretty trivial: http://git.kernel.dk/cgit/linux-block/commit/?h=mq-inflight&id=b841804f826df072f706ae86d0eb533342f0297a And updated the branch here: http://git.kernel.dk/cgit/linux-block/log/?h=mq-inflight to include that, and be based on top of for-4.13/block. If you prefer just pulling a branch, pull: git://git.kernel.dk/linux-block mq-inflight -- Jens Axboe
