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@xxxxxxxxxxxxxxxxx>
>>> ---
>>> 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.
Also, IOPOLL+SQPOLL io_uring_try_cancel_requests() looks like
may fail (didn't double check it). Look again for IORING_SETUP_SQPOLL.
I'd rather recommend to go over all uses of IORING_SETUP_SQPOLL
and think whether it's flawed.
>
>
>>
>>> + else
>>> + tctx = req->task->io_uring;
>>> BUG_ON(!tctx);
>>> BUG_ON(!tctx->io_wq);
>>
>> [snip]
>>
>
--
Pavel Begunkov
