Re: [dm-devel] [PATCH 1/1] block: Convert hd_struct in_flight from atomic to percpu

[Jens Axboe <axboe@kernel.dk>]

On 06/30/2017 10:59 PM, Jens Axboe wrote:
> 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@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@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.
>>>
>>> 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

Now:

http://git.kernel.dk/cgit/linux-block/commit/?h=mq-inflight&id=87f73ef2b9edb6834001df8f7cb48c7a116e8cd3

> 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

I've tested it, and made a few adjustments and fixes. The branches are
the same, just rebased. Works fine for me, even with partitions now.

Let me know how it works for you.

-- 
Jens Axboe