Re: [PATCH 1/2] nvme: pci: simplify timeout handling

[Ming Lei <ming.lei@redhat.com>]

On Fri, Apr 27, 2018 at 11:51:57AM -0600, Keith Busch wrote:
> On Thu, Apr 26, 2018 at 08:39:55PM +0800, Ming Lei wrote:
> > +/*
> > + * This one is called after queues are quiesced, and no in-fligh timeout
> > + * and nvme interrupt handling.
> > + */
> > +static void nvme_pci_cancel_request(struct request *req, void *data,
> > +		bool reserved)
> > +{
> > +	/* make sure timed-out requests are covered too */
> > +	if (req->rq_flags & RQF_MQ_TIMEOUT_EXPIRED) {
> > +		req->aborted_gstate = 0;
> > +		req->rq_flags &= ~RQF_MQ_TIMEOUT_EXPIRED;
> > +	}
> > +
> > +	nvme_cancel_request(req, data, reserved);
> > +}
> 
> I don't know about this. I feel like blk-mq owns these flags and LLD's
> shouldn't require such knowledge of their implementation details.

If driver returns BLK_EH_NOT_HANDLED from .timeout(), the driver may
need to deal with this flag.

But it won't be necessary to use this flag here, and it can be done by
clearing 'req->aborted_gstate' always.

> 
> I understand how the problems are happening a bit better now. It used
> to be that blk-mq would lock an expired command one at a time, so when
> we had a batch of IO timeouts, the driver was able to complete all of
> them inside a single IO timeout handler.
> 
> That's not the case anymore, so the driver is called for every IO
> timeout even though if it reaped all the commands at once.

Actually there isn't the case before, even for legacy path, one .timeout()
handles one request only.

> 
> IMO, the block layer should allow the driver to complete all timed out
> IOs at once rather than go through ->timeout() for each of them and
> without knowing about the request state details. Much of the problems
> would go away in that case.

That need to change every driver, looks not easy to do, also not sure
if other drivers need this way.

> 
> But I think much of this can be fixed by just syncing the queues in the
> probe work, and may be a more reasonable bug-fix for 4.17 rather than
> rewrite the entire timeout handler. What do you think of this patch? It's
> an update of one I posted a few months ago, but uses Jianchao's safe
> namespace list locking.
> 
> ---
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index a3771c5729f5..198b4469c3e2 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -3562,12 +3562,25 @@ void nvme_start_queues(struct nvme_ctrl *ctrl)
>  	struct nvme_ns *ns;
>  
>  	down_read(&ctrl->namespaces_rwsem);
> -	list_for_each_entry(ns, &ctrl->namespaces, list)
> +	list_for_each_entry(ns, &ctrl->namespaces, list) {
>  		blk_mq_unquiesce_queue(ns->queue);
> +		blk_mq_kick_requeue_list(ns->queue);
> +	}
>  	up_read(&ctrl->namespaces_rwsem);
>  }
>  EXPORT_SYMBOL_GPL(nvme_start_queues);
>  
> +void nvme_sync_queues(struct nvme_ctrl *ctrl)
> +{
> +	struct nvme_ns *ns;
> +
> +	down_read(&ctrl->namespaces_rwsem);
> +	list_for_each_entry(ns, &ctrl->namespaces, list)
> +		blk_sync_queue(ns->queue);
> +	up_read(&ctrl->namespaces_rwsem);
> +}
> +EXPORT_SYMBOL_GPL(nvme_sync_queues);
> +
>  int nvme_reinit_tagset(struct nvme_ctrl *ctrl, struct blk_mq_tag_set *set)
>  {
>  	if (!ctrl->ops->reinit_request)
> diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
> index 7ded7a51c430..e62273198725 100644
> --- a/drivers/nvme/host/nvme.h
> +++ b/drivers/nvme/host/nvme.h
> @@ -402,6 +402,7 @@ int nvme_sec_submit(void *data, u16 spsp, u8 secp, void *buffer, size_t len,
>  void nvme_complete_async_event(struct nvme_ctrl *ctrl, __le16 status,
>  		union nvme_result *res);
>  
> +void nvme_sync_queues(struct nvme_ctrl *ctrl);
>  void nvme_stop_queues(struct nvme_ctrl *ctrl);
>  void nvme_start_queues(struct nvme_ctrl *ctrl);
>  void nvme_kill_queues(struct nvme_ctrl *ctrl);
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index fbc71fac6f1e..a2f3ad105620 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -2303,6 +2303,7 @@ static void nvme_reset_work(struct work_struct *work)
>  	 */
>  	if (dev->ctrl.ctrl_config & NVME_CC_ENABLE)
>  		nvme_dev_disable(dev, false);
> +	nvme_sync_queues(&dev->ctrl);

This sync may be raced with one timed-out request, which may be handled
as BLK_EH_HANDLED or BLK_EH_RESET_TIMER, so the above sync queues can't
work reliably. 

There are more issues except for the above one if disabling controller
and resetting it are run in different context:

1) inside resetting, nvme_dev_disable() may be called, but this way
may cause double completion on the previous timed-out request.

2) the timed-out request can't be covered by nvme_dev_disable(), and
this way is too tricky and easy to cause trouble.

IMO, it is much easier to avoid the above issues by handling controller
recover in one single thread. I will address all your comments and post
V3 for review.


Thanks,
Ming