Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Wed Aug 04 2010 - 18:21:56 EST

On Wed, 4 Aug 2010, Matthew Garrett wrote:

On Wed, Aug 04, 2010 at 12:29:36PM -0700, david@xxxxxxx wrote:

why would you trust it to take a wakelock, but not trust it the rest of
the time?

Because I trust that when the application author says "I explicitly need
the machine to stay awake" that they mean it, whereas I don't trust the
application author to write an application that avoids consuming
background CPU. The distinction is pretty important.

on the other hand, making an application avoid consuming inappropriate background resources helps everywhere. an explicit "don't let the machine sleep" only works if you are trusted by a system that implements this flag.

yes, it is painful to make the change, but the end result is better (and there are more tools out there to figure things out)

in my proposal I'm saying that if you would trust the application to take
a wakelock, you instead trust it to be sane in the rest of it's power
activity (avoiding polling, etc) and so you consider it for sleep

When we say "trust", we're not using the same meaning as we do with
security. Yes, it's possible that an application that can block suspend
will do so at inopportune times. But given that blocking suspend is an
explicit act it's much more likely that the developer will only use it
in reasonable ways, while it's still entirely plausible that the
application will generate unnecessary wakeups. Pretending otherwise is
unrealistic. I recently had to fix the fact that the kernel IPMI layer
would generate a constant 1000 wakeups a second even if it had an
interrupt-driven controller or was entirely idle.

I'm not sure I buy the distinction, but I'm not a maintainer so it's others you have to convince.

it's better to have one type of problem with one set of tools that document why the machine isn't sleeping, than to have orthoginal ways of influencing power management.

but if you have an application in the mid-level trust situation, go ahead and have it talk to a 'keepalive' daemon that is in the 'trusted' set and let the rest of the app run untrusted. As I noted elsewhere, the keepalive daemon would need very little in the way of resources and can implement much more complex policies than anyone is going to be willing to put in the kernel.

The second is that the incoming network packet causes
the kernel to take a wakelock that will be released once userspace has
processed the network packet. This ensures that at least one wakelock is
held for the entire relevant period of time.

how do you determine that userspace has processed the network packet so
that the kernel can release the wakelock (or is this one of the cases
where there is a timer related to the wakelock)

The current implementation uses a timer, but Rafael's implementation
should allow userspace to explicitly acknowledge it.

similar to the current implementation, the arrival of a packet could be counted as activity that keeps the system awake for a bit (your timeout)

two things here,

on the dirty networks that I see as common, refusing to sleep if network
packets are arriving will mean that you never go to sleep.

Cell networks typically have no background traffic, for obvious reasons.

but don't most new smartphones also connect up to wifi networks? those are FAR from quiet.

secondly, nothing stops the code doing the idle/suspend decision from
considering network activity. I would be surprised if there weren't
already options to support this today.

If you proxy every potential wakeup event through some central server
then this becomes much easier, but it's also a performance hit. The
alternative is that you poll for network activity, but that's a power

I'm not suggesting running all events through some central server (unless you count the kernel as that server), I'm saying that the decision that the system is idle and therefor can be stopped should be able to take this infomation into account, and if there's a race here, it should be a race that exists everywhere else, so there should be a general solution, not something specific to one use-case. (and definantly not something that requires all software to be modified and trusted to implement)

David Lang
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