On 7/19/2017 5:20 AM, Sagi Grimberg wrote:
>> Code is moving the completion queue doorbell after processing all completed
>> events and sending callbacks to the block layer on each iteration.
>>
>> This is causing a performance drop when a lot of jobs are queued towards
>> the HW. Move the completion queue doorbell on each loop instead and allow new
>> jobs to be queued by the HW.
>>
>> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
>> ---
>> drivers/nvme/host/pci.c | 5 ++---
>> 1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
>> index d10d2f2..33d9b5b 100644
>> --- a/drivers/nvme/host/pci.c
>> +++ b/drivers/nvme/host/pci.c
>> @@ -810,13 +810,12 @@ static void nvme_process_cq(struct nvme_queue *nvmeq)
>> while (nvme_read_cqe(nvmeq, &cqe)) {
>> nvme_handle_cqe(nvmeq, &cqe);
>> + nvme_ring_cq_doorbell(nvmeq);
>> consumed++;
>> }
>> - if (consumed) {
>> - nvme_ring_cq_doorbell(nvmeq);
>> + if (consumed)
>> nvmeq->cqe_seen = 1;
>> - }
>> }
>
> Agree with Keith that this is definitely not the way to go, it
> adds mmio operations in the hot path with very little gain (if
> at all).
>
Understood, different architectures might have different latency accessing the HW
registers. It might be expansive on some platform like you indicated and this change
would make it worse.
I'm doing a self NACK as well.
--
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
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