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

Jens Axboe
2018-07-17T21:49:52Z

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.

That said, we should still make it as efficient as we can, as per
previous discussions.

--
Jens Axboe