Re: [PATCH for-next] RDMA/hns: Bugfix for flush cqe in case softirq and multi-process

[Leon Romanovsky <leon@kernel.org>]

On Sat, Oct 12, 2019 at 11:53:36AM +0800, Liuyixian (Eason) wrote:
>
>
> On 2019/9/24 11:54, Liuyixian (Eason) wrote:
> >
> >
> > On 2019/9/23 13:01, Leon Romanovsky wrote:
> >> On Fri, Sep 20, 2019 at 11:55:56AM +0800, Liuyixian (Eason) wrote:
> >>>
> >>>
> >>> On 2019/9/11 21:17, Liuyixian (Eason) wrote:
> >>>>
> >>>>
> >>>> On 2019/9/10 15:52, Leon Romanovsky wrote:
> >>>>> On Tue, Sep 10, 2019 at 02:40:20PM +0800, Liuyixian (Eason) wrote:
> >>>>>>
> >>>>>>
> >>>>>> On 2019/9/8 16:03, Leon Romanovsky wrote:
> >>>>>>> On Thu, Sep 05, 2019 at 08:31:11PM +0800, Weihang Li wrote:
> >>>>>>>> From: Yixian Liu <liuyixian@huawei.com>
> >>>>>>>>
> >>>>>>>> Hip08 has the feature flush cqe, which help to flush wqe in workqueue
> >>>>>>>> (sq and rq) when error happened by transmitting producer index with
> >>>>>>>> mailbox to hardware. Flush cqe is emplemented in post send and recv
> >>>>>>>> verbs. However, under NVMe cases, these verbs will be called under
> >>>>>>>> softirq context, and it will lead to following calltrace with
> >>>>>>>> current driver as mailbox used by flush cqe can go to sleep.
> >>>>>>>>
> >>>>>>>> This patch solves this problem by using workqueue to do flush cqe,
> >>>>>>>
> >>>>>>> Unbelievable, almost every bug in this driver is solved by introducing
> >>>>>>> workqueue. You should fix "sleep in flush path" issue and not by adding
> >>>>>>> new workqueue.
> >>>>>>>
> >>>>>> Hi Leon,
> >>>>>>
> >>>>>> Thanks for the comment.
> >>>>>> Up to now, for hip08, only one place use workqueue in hns_roce_hw_v2.c
> >>>>>> where for irq prints.
> >>>>>
> >>>>> Thanks to our lack of desire to add more workqueues and previous patches
> >>>>> which removed extra workqueues from the driver.
> >>>>>
> >>>> Thanks, I see.
> >>>>
> >>>>>>
> >>>>>> The solution for flush cqe in this patch is as follow:
> >>>>>> While flush cqe should be implement, the driver should modify qp to error state
> >>>>>> through mailbox with the newest product index of sq and rq, the hardware then
> >>>>>> can flush all outstanding wqes in sq and rq.
> >>>>>>
> >>>>>> That's the whole mechanism of flush cqe, also is the flush path. We can't
> >>>>>> change neither mailbox sleep attribute or flush cqe occurred in post send/recv.
> >>>>>> To avoid the calltrace of flush cqe in post verbs under NVMe softirq,
> >>>>>> use workqueue for flush cqe seems reasonable.
> >>>>>>
> >>>>>> As far as I know, there is no other alternative solution for this situation.
> >>>>>> I will be very grateful if you reminder me more information.
> >>>>>
> >>>>> ib_drain_rq/ib_drain_sq/ib_drain_qp????
> >>>>>
> >>>> Hi Leon,
> >>>>
> >>>> I think these interfaces are designed for application to check that all wqes
> >>>> have been processed by hardware, so called drain or flush. However, it is not
> >>>> the same as the flush in this patch. The solution in this patch is used
> >>>> to help the hardware generate flush cqes for outstanding wqes while qp error.
> >>>>
> >>> Hi Leon,
> >>>
> >>> What's your opinion about above? Do you have any further comments?
> >>
> >> My opinion didn't change, you need to read discussions about ib_drain_*()
> >> functions, how and why they were introduced. It is a way to go.
> >>
> >> Thanks
> >
> > Hi Leon,
> >
> > Thanks a lot! I will dig those functions for my problem.
> >
>
> Hi Leon,
>
> I have analysis the mechanism of ib_drain_(qp, sq, rq), that's okay to use
> it instead of our flush cqe as both of them are calling modify qp to error
> state in flush path.
>
> However, both ib_drain_* and flush cqe will face the same problem as declared
> in previous emails, that is, in NVME case, post verbs will be called under
> **softirq**, which will result to calltrace as mailbox used in modify qp
> (flush path) can sleep, this is not allowed under softirq.
>
> Thus, to resolve above calltrace (sleep in softirq), using workqueue as in
> this patch seems is a reasonable solution regardless of ib_drain_qp or
> flush cqe is called in the workqueue.
>
> I think it is not a good idea to fix sleep in flush path (actually referred
> to mailbox used in modify qp) as the mailbox is such a mature mechanism.

No, it is not reasonable solution.

>
> Thanks.
>