[dm-devel] [PATCH 1/1] block: Convert hd_struct in_flight from atomic to percpu
Brian King
brking at linux.vnet.ibm.com
Fri Jun 30 13:05:06 UTC 2017
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 at kernel.dk> 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 at kernel.dk> 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
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