Re: [PATCH v2] scsi: ses: don't get power status of SES device slot on probe

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Apr 5, 2017 at 8:18 AM, Mauricio Faria de Oliveira
<mauricfo@linux.vnet.ibm.com> wrote:
> The commit 08024885a2a3 ("ses: Add power_status to SES device slot")
> introduced the 'power_status' attribute to enclosure components and
> the associated callbacks.
>
> There are 2 callbacks available to get the power status of a device:
> 1) ses_get_power_status() for 'struct enclosure_component_callbacks'
> 2) get_component_power_status() for the sysfs device attribute
> (these are available for kernel-space and user-space, respectively.)
>
> However, despite both methods being available to get power status
> on demand, that commit also introduced a call to get power status
> in ses_enclosure_data_process().
>
> This dramatically increased the total probe time for SCSI devices
> on larger configurations, because ses_enclosure_data_process() is
> called several times during the SCSI devices probe and loops over
> the component devices (but that is another problem, another patch).
>
> That results in a tremendous continuous hammering of SCSI Receive
> Diagnostics commands to the enclosure-services device, which does
> delay the total probe time for the SCSI devices __significantly__:
>
>   Originally, ~34 minutes on a system attached to ~170 disks:
>
>     [ 9214.490703] mpt3sas version 13.100.00.00 loaded
>     ...
>     [11256.580231] scsi 17:0:177:0: qdepth(16), tagged(1), simple(0),
>                    ordered(0), scsi_level(6), cmd_que(1)
>
>   With this patch, it decreased to ~2.5 minutes -- a 13.6x faster
>
>     [ 1002.992533] mpt3sas version 13.100.00.00 loaded
>     ...
>     [ 1151.978831] scsi 11:0:177:0: qdepth(16), tagged(1), simple(0),
>                    ordered(0), scsi_level(6), cmd_que(1)
>
> Back to the commit discussion.. on the ses_get_power_status() call
> introduced in ses_enclosure_data_process(): impact of removing it.
>
> That may possibly be in place to initialize the power status value
> on device probe.  However, those 2 functions available to retrieve
> that value _do_ automatically refresh/update it.  So the potential
> benefit would be a direct access of the 'power_status' field which
> does not use the callbacks...
>
> But the only reader of 'struct enclosure_component::power_status'
> is the get_component_power_status() callback for sysfs attribute,
> and it _does_ check for and call the .get_power_status callback,
> (which indeed is defined and implemented by that commit), so the
> power status value is, again, automatically updated.
>
> So, the remaining potential for a direct/non-callback access to
> the power_status attribute would be out-of-tree modules -- well,
> for those, if they are for whatever reason interested in values
> that are set during device probe and not up-to-date by the time
> they need it.. well, that would be curious.
>
> Well, to handle that more properly, set the initial power state
> value to '-1' (i.e., uninitialized) instead of '1' (power 'on'),
> and check for it in that callback which may do an direct access
> to the field value _if_ a callback function is not defined.
>
> Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
> Fixes: 08024885a2a3 ("ses: Add power_status to SES device slot")

Reviewed-by: Dan Williams <dan.j.williams@intel.com>