Re: [PATCH RFC 5.13 2/2] io_uring: submit sqes in the original context when waking up sqthread

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/5/21 2:10 PM, Hao Xu wrote:
> 在 2021/4/30 上午6:10, Pavel Begunkov 写道:
>> On 4/29/21 9:44 AM, Hao Xu wrote:
>>> 在 2021/4/28 下午10:34, Pavel Begunkov 写道:
>>>> On 4/28/21 2:32 PM, Hao Xu wrote:
>>>>> sqes are submitted by sqthread when it is leveraged, which means there
>>>>> is IO latency when waking up sqthread. To wipe it out, submit limited
>>>>> number of sqes in the original task context.
>>>>> Tests result below:
>>>>
>>>> Frankly, it can be a nest of corner cases if not now then in the future,
>>>> leading to a high maintenance burden. Hence, if we consider the change,
>>>> I'd rather want to limit the userspace exposure, so it can be removed
>>>> if needed.
>>>>
>>>> A noticeable change of behaviour here, as Hao recently asked, is that
>>>> the ring can be passed to a task from a completely another thread group,
>>>> and so the feature would execute from that context, not from the
>>>> original/sqpoll one.
>>>>
>>>> Not sure IORING_ENTER_SQ_DEPUTY knob is needed, but at least can be
>>>> ignored if the previous point is addressed.
>>>>
>>>>>
>>>>> 99th latency:
>>>>> iops\idle    10us    60us    110us    160us    210us    260us    310us    360us    410us    460us    510us
>>>>> with this patch:
>>>>> 2k          13    13    12    13    13    12    12    11    11    10.304    11.84
>>>>> without this patch:
>>>>> 2k          15    14    15    15    15    14    15    14    14    13    11.84
>>>>
>>>> Not sure the second nine describes it well enough, please can you
>>>> add more data? Mean latency, 50%, 90%, 99%, 99.9%, t-put.
>>>>
>>>> Btw, how happened that only some of the numbers have fractional part?
>>>> Can't believe they all but 3 were close enough to integer values.
>>>>
>>>>> fio config:
>>>>> ./run_fio.sh
>>>>> fio \
>>>>> --ioengine=io_uring --sqthread_poll=1 --hipri=1 --thread=1 --bs=4k \
>>>>> --direct=1 --rw=randread --time_based=1 --runtime=300 \
>>>>> --group_reporting=1 --filename=/dev/nvme1n1 --sqthread_poll_cpu=30 \
>>>>> --randrepeat=0 --cpus_allowed=35 --iodepth=128 --rate_iops=${1} \
>>>>> --io_sq_thread_idle=${2}
>>>>>
>>>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>>>> ---
>>>>>    fs/io_uring.c                 | 29 +++++++++++++++++++++++------
>>>>>    include/uapi/linux/io_uring.h |  1 +
>>>>>    2 files changed, 24 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>> index 1871fad48412..f0a01232671e 100644
>>>>> --- a/fs/io_uring.c
>>>>> +++ b/fs/io_uring.c
>>>>> @@ -1252,7 +1252,12 @@ static void io_queue_async_work(struct io_kiocb *req)
>>>>>    {
>>>>>        struct io_ring_ctx *ctx = req->ctx;
>>>>>        struct io_kiocb *link = io_prep_linked_timeout(req);
>>>>> -    struct io_uring_task *tctx = req->task->io_uring;
>>>>> +    struct io_uring_task *tctx = NULL;
>>>>> +
>>>>> +    if (ctx->sq_data && ctx->sq_data->thread)
>>>>> +        tctx = ctx->sq_data->thread->io_uring;
>>>>
>>>> without park it's racy, sq_data->thread may become NULL and removed,
>>>> as well as its ->io_uring.
>>> I now think that it's ok to queue async work to req->task->io_uring. I
>>> look through all the OPs, seems only have to take care of async_cancel:
>>>
>>> io_async_cancel(req) {
>>>     cancel from req->task->io_uring;
>>>     cancel from ctx->tctx_list
>>> }
>>>
>>> Given req->task is 'original context', the req to be cancelled may in
>>> ctx->sq_data->thread->io_uring's iowq. So search the req from
>>> sqthread->io_uring is needed here. This avoids overload in main code
>>> path.
>>> Did I miss something else?
>>
>> It must be req->task->io_uring, otherwise cancellations will
>> be broken. And using it should be fine in theory because io-wq/etc.
>> should be set up by io_uring_add_task_file()
>>
>>
>> One more problem to the pile is io_req_task_work_add() and notify
>> mode it choses. Look for IORING_SETUP_SQPOLL in the function.
> How about:
> notify = TWA_SIGNAL
> if ( (is sq mode) and (sqd->thread == NULL or == req->task))
>    notify = TWA_NONE;

notify = (sqd && tsk == sqd->thread) ? TWA_NONE : TWA_SIGNAL;

Like that? Should work


>> Also, IOPOLL+SQPOLL io_uring_try_cancel_requests() looks like
>> may fail (didn't double check it). Look again for IORING_SETUP_SQPOLL.
>>
> I've excluded IOPOLL. This change will only affect SQPOLL mode.
>> I'd rather recommend to go over all uses of IORING_SETUP_SQPOLL
>> and think whether it's flawed.
> I'm working on this. (no obvious problem through eyes, will put the code
> change on more tests)
>>
>>>
>>>
>>>>
>>>>> +    else
>>>>> +        tctx = req->task->io_uring;
>>>>>          BUG_ON(!tctx);
>>>>>        BUG_ON(!tctx->io_wq);
>>>>
>>>> [snip]
>>>>
>>>
>>
> 

-- 
Pavel Begunkov