On Mon, Mar 10, 2014 at 1:43 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> On Fri, Mar 07, 2014 at 06:52:06PM -0800, Dan Williams wrote:
>> From: Dan Williams <dan.j.williams@xxxxxxxxx>
>>
>> async_schedule() sd resume work to allow disks and other devices to
>> resume in parallel.
>>
>> This moves the entirety of scsi_device resume to an async context to
>> ensure that scsi_device_resume() remains ordered with respect to the
>> completion of the start/stop command. For the duration of the resume,
>> new command submissions (that do not originate from the scsi-core) will
>> be deferred (BLKPREP_DEFER).
>>
>> It adds a new ASYNC_DOMAIN_EXCLUSIVE(scsi_sd_pm_domain) as a container
>> of these operations. Like scsi_sd_probe_domain it is flushed at
>> sd_remove() time to ensure async ops do not continue past the
>> end-of-life of the sdev. The implementation explicitly refrains from
>> reusing scsi_sd_probe_domain directly for this purpose as it is flushed
>> at the end of dpm_resume(), potentially defeating some of the benefit.
>> Given sdevs are quiesced it is permissible for these resume operations
>> to bleed past the async_synchronize_full() calls made by the driver
>> core.
>>
>> We defer the resolution of which pm callback to call until
>> scsi_dev_type_{suspend|resume} time and guarantee that the callback
>> parameter is never NULL. With this in place the type of resume
>> operation is encoded in the async function identifier.
>>
>> Inspired by Todd's analysis and initial proposal [2]:
>> https://01.org/suspendresume/blogs/tebrandt/2013/hard-disk-resume-optimization-simpler-approach
>
> The only thing which is a bit concerning is that this doesn't have any
> throttling mechanism for simultaneous wakeups. Would this be able to
> blow up the PSU if used on a machine with a lot of spindles?
Good point. The primary benefit is completing userspace resume
without needlessly waiting for the disk. For now I think it would be
enough to have a mutex to maintain one disk at a time. We can follow
on later with something more complex to enable a max simultaneous
spin-up tunable.
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