Re: NVMe Poll CQ on timeout

[Keith Busch <kbusch@kernel.org>]

On Thu, Sep 19, 2019 at 01:47:50PM +0000, Bharat Kumar Gogada wrote:
> Hi All,
> 
> We are testing NVMe cards on ARM64 platform, the card uses MSI-X interrupts.
> We are hitting following case in drivers/nvme/host/pci.c
> /*
>          * Did we miss an interrupt?
>          */
>         if (__nvme_poll(nvmeq, req->tag)) {
>                 dev_warn(dev->ctrl.device,
>                          "I/O %d QID %d timeout, completion polled\n",
>                          req->tag, nvmeq->qid);
>                 return BLK_EH_DONE;
>         }
> 
> Can anyone tell when does nvme_timeout gets invoked ?

Timeout is invoked when the driver didn't see a completion to a
submitted command.

> In what cases we see this interrupt miss ?

That usually happens for one of two reasons:

 1. The device didn't send any MSIx message for a CQE

 2. The device sent the MSIx message before posting the CQE

I've also seen h/w errata where the MSIx and CQE are re-ordered, which
can also lead to this.

A hardware trace would provide the most detailed view of what's
happening. You might be able to infer if you carefully account for
commands sent, interrupts received, and spurious interrupts detected.

> We are seeing this issue only for reads with following fio command 
> fio --name=randwrite --ioengine=libaio --iodepth=1 --rw=randread --bs=128k --direct=0 \
> --size=128M --numjobs=3 --group_reporting --filename=/dev/nvme0n1
> 
> We are not seeing issue with --rw=randwrite for same size.
> 
> Please let us know what can cause this issue.