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. -- Jens Axboe
