Re: [PATCH v2 10/11] megaraid_sas: Use Block layer API to check SCSI device in-flight IO requests

[John Garry <john.garry@huawei.com>]

> 
>     Is blk_mq_hw_ctx.nr_active really the same as scsi_device.device_busy?
> 
> *Both of them are not the same but it serves our purpose to get the 
> number of outstanding io requests. Please refer below link for more 
> details:*
> 
> https://lore.kernel.org/linux-scsi/20191105002334.GA11436@ming.t460p/

Thanks for the pointer, but there did not seem to be a conclusion there: 
https://lore.kernel.org/linux-scsi/20191105002334.GA11436@ming.t460p/

Anyway, if we move to exposing multiple HW queues in the megaraid SAS 
driver:

  host->nr_hw_queues = instance->msix_vectors -
                       instance->low_latency_index_start;

Then hctx->nr_active will no longer be the total active requests per 
host, but rather per hctx.

In addition, hctx->nr_active will no longer be properly maintained, as 
it would be based on the hctx HW queue actually being used by the LLDD 
for that request, which is not always true now. That is because in 
megasas_get_msix_index() a judgement may be made to use a low-latency HW 
queue instead:

static inline void
megasas_get_msix_index(struct megasas_instance *instance,
		       struct scsi_cmnd *scmd,
		       struct megasas_cmd_fusion *cmd,
		       u8 data_arms)
{
...

sdev_busy = atomic_read(&hctx->nr_active);

if (instance->perf_mode == MR_BALANCED_PERF_MODE &&
     sdev_busy > (data_arms * MR_DEVICE_HIGH_IOPS_DEPTH))
	cmd->request_desc->SCSIIO.MSIxIndex =
			mega_mod64(...),
	else if (instance->msix_load_balance)
		cmd->request_desc->SCSIIO.MSIxIndex =
			(mega_mod64(...),
				instance->msix_vectors));

Will this make a difference? I am not sure. Maybe, on this basis, 
magaraid sas is not a good candidate to change to expose multiple queues.

Ignoring that for a moment, since we no longer keep a host busy count, 
and I figure that we don't want to back to using 
scsi_device.device_busy, is the judgement of hctx->nr_active ok to use 
to decide whether to use these performance queues?

Thanks,
John