Re: [PATCH RFC V2 3/3] scsi_mq: enable runtime PM

[Hannes Reinecke <hare@suse.de>]

On 07/18/2018 02:06 PM, Ming Lei wrote:
> On Wed, Jul 18, 2018 at 5:49 AM, Jens Axboe <axboe@kernel.dk> wrote:
>> On 7/17/18 9:38 AM, Ming Lei wrote:
>>> On Tue, Jul 17, 2018 at 03:34:35PM +0000, Bart Van Assche wrote:
>>>> On Tue, 2018-07-17 at 23:30 +0800, Ming Lei wrote:
>>>>> On Tue, Jul 17, 2018 at 03:24:11PM +0200, Christoph Hellwig wrote:
>>>>>> On Fri, Jul 13, 2018 at 04:06:02PM +0800, Ming Lei wrote:
>>>>>>> Usually SCSI supports runtime PM, so pass BLK_MQ_F_SUPPORT_RPM to blk-mq
>>>>>>> core for enabling block runtime PM.
>>>>>>
>>>>>> I still think enabling this unconditionally for any SCSI device was
>>>>>> a mistake, and it is even more so for blk-mq.
>>>>>>
>>>>>> Please only opt in for ufs, ATA first, adding others if wanted by
>>>>>> maintainers.
>>>>>
>>>>> No, this way will cause regression because runtime PM works for
>>>>> all sd/sr device actually, and it isn't related with scsi host.
>>>>
>>>> For which SCSI devices other than ufs and ATA do we need PM support?
>>>
>>> As I said, it is any sd/sr device, which can be put down by runtime PM
>>> via START_STOP command if it isn't used for a while.
>>
>> Christoph is basically echoing my concerns. Why don't we just enable
>> it on slower devices, similarly to what we do for adding
>> randomness? Nobody wants to pay this overhead for faster devices,
>> since most people won't care.
> 
> IMO the problem isn't related with slow or quick device, it is related with
> the system, especially when it cares about power consumption, such as
> mobile phone, or laptop or servers with lots of disks attached. And we know
> it is often to see some fast disks shipped in laptop, such as NVMe, or other
> SSD.
> 
But those typically have dedicated (transport/driver) mechanism to put
the device to sleep, _and_ START STOP UNIT will be emulated for those
devices anyway.
I'd rather advocate to move runtime PM into the individual
subsystems/drivers, and _not_ abuse START STOP UNIT here.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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