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

James Bottomley
2010-06-03T14:55:00Z

On Thu, 2010-06-03 at 16:35 +0200, Thomas Gleixner wrote:
> On Thu, 3 Jun 2010, James Bottomley wrote:
>
> > 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)?
>
> It depends on the information type and for a lot of things we might
> get away without notifiers.
>
> The only real issue is when you need to get other cores out of their
> deep idle state to make a new constraint work. That's what we do with
> the DMA latency notifier right now.

But the only DMA latency notifier is cpuidle_latency_notifier. That
looks callable from atomic context, so we could have two chains: one
atomic and one not.

The only other notifier in use is the ieee80211_max_network_latency,
which uses mutexes, so does require user context.

James