Re: [PATCHv3 2/4] nvme/pci: Complete commands from primary handler

[Christoph Hellwig <hch@lst.de>]

On Tue, Dec 10, 2019 at 02:56:20AM +0900, Keith Busch wrote:
> The context switch to wake the threaded interrupt handler has enough
> overhead to harm latency for low queue depth workloads. Complete one cycle
> through the completion queue in the primary handler to ensure threaded
> interrupts have no disadvantage compared to not using threaded interrupts. Wake
> the thread only if more completions remain after processing the queue
> once since that indicates a higher queue depth workload that may trigger
> frequent interrupts which would be better completed in the handler thread
> with device interrupts disabled.
> 
> Completing requests from two different contexts is safe since nvme driver
> disables entry to the primary handler when it returns IRQ_WAKE_THREAD.
> 
> Signed-off-by: Keith Busch <kbusch@kernel.org>
> ---
>  drivers/nvme/host/pci.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index a0138e3ca0b9..e415fadf7331 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -1055,6 +1055,10 @@ static irqreturn_t nvme_irq_check(int irq, void *data)
>  	if (!nvme_cqe_pending(nvmeq))
>  		return IRQ_NONE;
>  
> +	nvme_irq(irq, data);

I think we can just open code nvme_irq here.  That also means we can
use the return value of nvme_process_cq instead of the extra
nvme_cqe_pending check above.  This also needs a good comment explaining
the scheme at least.

> +	if (!nvme_cqe_pending(nvmeq))
> +		return IRQ_HANDLED;

Including one explaining this check.

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