Re: [PATCH 0/8] Suspend block api (version 8)

Gross, Mark
2010-06-03T14:26:55Z

>-----Original Message-----
>From: James Bottomley [mailto:James.Bottomley@suse.de]
>Sent: Thursday, June 03, 2010 6:25 AM
>To: Alan Cox
>Cc: Gross, Mark; Florian Mickler; Arve Hjønnevåg; Neil Brown;
>tytso@mit.edu; Peter Zijlstra; LKML; Thomas Gleixner; Linux OMAP Mailing
>List; Linux PM; felipe.balbi@nokia.com
>Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)
>
>On Thu, 2010-06-03 at 11:03 +0100, Alan Cox wrote:
>> > [mtg: ] This has been a pain point for the PM_QOS implementation. They
>change the constrain back and forth at the transaction level of the i2c
>driver. The pm_qos code really wasn't made to deal with such hot path use,
>as each such change triggers a re-computation of what the aggregate qos
>request is.
>>
>> That should be trivial in the usual case because 99% of the time you can
>> hot path
>>
>> the QoS entry changing is the latest one
>> there have been no other changes
>> If it is valid I can use the cached previous aggregate I cunningly
>> saved in the top QoS entry when I computed the new one
>>
>> (ie most of the time from the kernel side you have a QoS stack)
>
>It's not just the list based computation: that's trivial to fix, as you
>say ... the other problem is the notifier chain, because that's blocking
>and could be long. Could we invoke the notifier through a workqueue?
>It doesn't seem to have veto power, so it's pure notification, does it
>matter if the notice is delayed (as long as it's in order)?
[mtg: ] true. The notifications "could be" done on as a scheduled work item
in most cases. I think there is only one user of the notification so far
any way. Most pm_qos users do a pole of the current value for whatever parameter they are interested in.

--mgross

>
>> > We've had a number of attempts at fixing this, but I think the proper
>fix is to bolt a "disable C-states > x" interface into cpu_idle that
>bypases pm_qos altogether. Or, perhaps add a new pm_qos API that does the
>equivalent operation, overriding whatever constraint is active.
>>
>> We need some of this anyway for deep power saving because there is
>> hardware which can't wake from soem states, which in turn means if that
>> device is active we need to be above the state in question.
>
>James
>