RE: [PATCH 10/15] megaraid_sas: IO throttling support

[Sumit Saxena <sumit.saxena@avagotech.com>]

> -----Original Message-----
> From: Tomas Henzl [mailto:thenzl@redhat.com]
> Sent: Tuesday, January 19, 2016 7:08 PM
> To: Sumit Saxena; jbottomley@parallels.com; hch@infradead.org;
> martin.petersen@oracle.com
> Cc: linux-scsi@vger.kernel.org; kashyap.desai@avagotech.com
> Subject: Re: [PATCH 10/15] megaraid_sas: IO throttling support
>
> On 18.12.2015 14:27, Sumit Saxena wrote:
> > This patch will add capability in driver to tell firmware that it can
> > throttle IOs in case Controller's Queue depth is downgraded post OFU
(Online
> firmware upgrade). This feature will ensure firmware can be downgraded
from
> higher queue depth to lower queue depth without needing system reboot.
> > Added throttling code in IO path of driver, in case OS tries to send
more IOs
> than post OFU firmware's queue depth.
> >
> > Signed-off-by: Sumit Saxena <sumit.saxena@avagotech.com>
> > Signed-off-by: Kashyap Desai <kashyap.desai@avagotech.com>
> > ---
> >  drivers/scsi/megaraid/megaraid_sas.h        |    6 ++++--
> >  drivers/scsi/megaraid/megaraid_sas_fusion.c |    7 +++++++
> >  2 files changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/scsi/megaraid/megaraid_sas.h
> > b/drivers/scsi/megaraid/megaraid_sas.h
> > index 4595ef4..9d2b3da 100644
> > --- a/drivers/scsi/megaraid/megaraid_sas.h
> > +++ b/drivers/scsi/megaraid/megaraid_sas.h
> > @@ -1537,7 +1537,8 @@ union megasas_sgl_frame {  typedef union
> > _MFI_CAPABILITIES {
> >  	struct {
> >  #if   defined(__BIG_ENDIAN_BITFIELD)
> > -		u32     reserved:21;
> > +		u32     reserved:20;
> > +		u32     support_qd_throttling:1;
> >  		u32     support_fp_rlbypass:1;
> >  		u32     support_vfid_in_ioframe:1;
> >  		u32     support_ext_io_size:1;
> > @@ -1561,7 +1562,8 @@ typedef union _MFI_CAPABILITIES {
> >  		u32     support_ext_io_size:1;
> >  		u32     support_vfid_in_ioframe:1;
> >  		u32     support_fp_rlbypass:1;
> > -		u32     reserved:21;
> > +		u32     support_qd_throttling:1;
> > +		u32     reserved:20;
> >  #endif
> >  	} mfi_capabilities;
> >  	__le32		reg;
> > diff --git a/drivers/scsi/megaraid/megaraid_sas_fusion.c
> > b/drivers/scsi/megaraid/megaraid_sas_fusion.c
> > index 7cc7806..1248c7a 100644
> > --- a/drivers/scsi/megaraid/megaraid_sas_fusion.c
> > +++ b/drivers/scsi/megaraid/megaraid_sas_fusion.c
> > @@ -801,6 +801,7 @@ megasas_ioc_init_fusion(struct megasas_instance
> *instance)
> >  	if (!dual_qdepth_disable)
> >  		drv_ops->mfi_capabilities.support_ext_queue_depth = 1;
> >
> > +	drv_ops->mfi_capabilities.support_qd_throttling = 1;
> >  	/* Convert capability to LE32 */
> >  	cpu_to_le32s((u32
> > *)&init_frame->driver_operations.mfi_capabilities);
> >
> > @@ -2182,6 +2183,12 @@ megasas_build_and_issue_cmd_fusion(struct
> megasas_instance *instance,
> >  			atomic_inc(&instance->ldio_outstanding);
> >  	}
> >
> > +	if (atomic_read(&instance->fw_outstanding) >=
> > +			instance->host->can_queue) {
> > +		dev_err(&instance->pdev->dev, "Throttle IOs beyond
Controller
> queue depth\n");
> > +		return SCSI_MLQUEUE_HOST_BUSY;
> > +	}
>
> Same as in the previous patch, this this test above won't you protect
when
> several processes read the same value in parallel. In addition to that,
when the
> scsi layer knows the new smaller can_queue value it will not queue new
> commands above the limit.

Agree that SCSI mid layer itself will not send IOs beyond can_queue value
and we can remove this code. This code was just extra check for older
kernels- if IOs returned to SCSI mid layer by driver with DID_RESET status
does not check updated can_queue value(not sure if it was always taken
care of so added this code).
>
> > +
> >  	cmd = megasas_get_cmd_fusion(instance, scmd->request->tag);
> >
> >  	index = cmd->index;