[dm-devel] [PATCH 1/1] block: Convert hd_struct in_flight from atomic to percpu
Ming Lei
tom.leiming at gmail.com
Fri Jun 30 01:20:11 UTC 2017
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), :-(
Thanks,
Ming Lei
More information about the dm-devel
mailing list