Re: [PATCH for-next v3 0/4] fixed-buffer for uring-cmd/passthrough

Kanchan Joshi
2022-09-03T09:44:56Z

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On Fri, Sep 02, 2022 at 03:25:33PM -0600, Jens Axboe wrote:
>On 9/2/22 1:32 PM, Jens Axboe wrote:
>> On 9/2/22 12:46 PM, Kanchan Joshi wrote:
>>> On Fri, Sep 02, 2022 at 10:32:16AM -0600, Jens Axboe wrote:
>>>> On 9/2/22 10:06 AM, Jens Axboe wrote:
>>>>> On 9/2/22 9:16 AM, Kanchan Joshi wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Currently uring-cmd lacks the ability to leverage the pre-registered
>>>>>> buffers. This series adds the support in uring-cmd, and plumbs
>>>>>> nvme passthrough to work with it.
>>>>>>
>>>>>> Using registered-buffers showed peak-perf hike from 1.85M to 2.17M IOPS
>>>>>> in my setup.
>>>>>>
>>>>>> Without fixedbufs
>>>>>> *****************
>>>>>> # taskset -c 0 t/io_uring -b512 -d128 -c32 -s32 -p0 -F1 -B0 -O0 -n1 -u1 /dev/ng0n1
>>>>>> submitter=0, tid=5256, file=/dev/ng0n1, node=-1
>>>>>> polled=0, fixedbufs=0/0, register_files=1, buffered=1, QD=128
>>>>>> Engine=io_uring, sq_ring=128, cq_ring=128
>>>>>> IOPS=1.85M, BW=904MiB/s, IOS/call=32/31
>>>>>> IOPS=1.85M, BW=903MiB/s, IOS/call=32/32
>>>>>> IOPS=1.85M, BW=902MiB/s, IOS/call=32/32
>>>>>> ^CExiting on signal
>>>>>> Maximum IOPS=1.85M
>>>>>
>>>>> With the poll support queued up, I ran this one as well. tldr is:
>>>>>
>>>>> bdev (non pt)??? 122M IOPS
>>>>> irq driven??? 51-52M IOPS
>>>>> polled??????? 71M IOPS
>>>>> polled+fixed??? 78M IOPS
>>>
>>> except first one, rest three entries are for passthru? somehow I didn't
>>> see that big of a gap. I will try to align my setup in coming days.
>>
>> Right, sorry it was badly labeled. First one is bdev with polling,
>> registered buffers, etc. The others are all the passthrough mode. polled
>> goes to 74M with the caching fix, so it's about a 74M -> 82M bump using
>> registered buffers with passthrough and polling.
>>
>>>> polled+fixed??? 82M
>>>>
>>>> I suspect the remainder is due to the lack of batching on the request
>>>> freeing side, at least some of it. Haven't really looked deeper yet.
>>>>
>>>> One issue I saw - try and use passthrough polling without having any
>>>> poll queues defined and it'll stall just spinning on completions. You
>>>> need to ensure that these are processed as well - look at how the
>>>> non-passthrough io_uring poll path handles it.
>>>
>>> Had tested this earlier, and it used to run fine. And it does not now.
>>> I see that io are getting completed, irq-completion is arriving in nvme
>>> and it is triggering task-work based completion (by calling
>>> io_uring_cmd_complete_in_task). But task-work never got called and
>>> therefore no completion happened.
>>>
>>> io_uring_cmd_complete_in_task -> io_req_task_work_add -> __io_req_task_work_add
>>>
>>> Seems task work did not get added. Something about newly added
>>> IORING_SETUP_DEFER_TASKRUN changes the scenario.
>>>
>>> static inline void __io_req_task_work_add(struct io_kiocb *req, bool allow_local)
>>> {
>>> ?????? struct io_uring_task *tctx = req->task->io_uring;
>>> ?????? struct io_ring_ctx *ctx = req->ctx;
>>> ?????? struct llist_node *node;
>>>
>>> ?????? if (allow_local && ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
>>> ?????????????? io_req_local_work_add(req);
>>> ?????????????? return;
>>> ?????? }
>>> ????....
>>>
>>> To confirm, I commented that in t/io_uring and it runs fine.
>>> Please see if that changes anything for you? I will try to find the
>>> actual fix tomorow.
>>
>> Ah gotcha, yes that actually makes a lot of sense. I wonder if regular
>> polling is then also broken without poll queues if
>> IORING_SETUP_DEFER_TASKRUN is set. It should be, I'll check into
>> io_iopoll_check().
>
>A mix of fixes and just cleanups, here's what I got.

Thanks, this looks much better. Just something to discuss on the fix
though. Will use other thread for that.

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