>-----Original Message----- >From: James Bottomley [mailto:James.Bottomley@xxxxxxx] >Sent: Thursday, June 03, 2010 6:25 AM >To: Alan Cox >Cc: Gross, Mark; Florian Mickler; Arve Hjønnevåg; Neil Brown; >tytso@xxxxxxx; Peter Zijlstra; LKML; Thomas Gleixner; Linux OMAP Mailing >List; Linux PM; felipe.balbi@xxxxxxxxx >Subject: Re: [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 > _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
