Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Mon Aug 02 2010 - 05:08:27 EST

On Mon, 2 Aug 2010, Florian Mickler wrote:

On Mon, 2 Aug 2010 00:02:04 -0700 (PDT)
david@xxxxxxx wrote:

On Mon, 2 Aug 2010, Florian Mickler wrote:

On Mon, 2 Aug 2010 08:40:03 +0200
Florian Mickler <florian@xxxxxxxxxxx> wrote:

On Sun, 1 Aug 2010 23:06:08 -0700 (PDT)
david@xxxxxxx wrote:

On Mon, 2 Aug 2010, Florian Mickler wrote:

On Sun, 1 Aug 2010 22:06:34 -0700 (PDT)
david@xxxxxxx wrote:

On Sun, 1 Aug 2010, Arjan van de Ven wrote:

I'm a little worried that this whole "I need to block suspend" is
temporary. Yes today there is silicon from ARM and Intel where suspend
is a heavy operation, yet at the same time it's not all THAT heavy
anymore.... at least on the Intel side it's good enough to use pretty
much all the time (when the screen is off for now, but that's a memory
controller issue more than anything else). I'm pretty sure the ARM guys
will not be far behind.

remember that this 'block suspend' is really 'block overriding the fact
that there are still runable processes and suspending anyway"

having it labeled as 'suspend blocker' or even 'wakelock' makes it sound
as if it blocks any attempt to suspend, and I'm not sure that's what's
really intended. Itsounds like the normal syspend process would continue
to work, just this 'ignore if these other apps are busy' mode of operation
would not work.

which makes me wonder, would it be possible to tell the normal idle
detection mechanism to ignore specific processes when deciding if it
should suspend or not? how about only considering processes in one cgroup
when deciding to suspend and ignoring all others?

David Lang

We then get again to the "runnable tasks" problem that was
discussed earlier... the system get's "deadlock-prone" if a subset of
tasks is not run.
Interprocess dependencies are not so easy to get right in general.

I'm not suggesting that you don't run the 'untrusted' tasks, just that you
don't consider them when deciding if the system can suspend or not. if the
system is awake, everything runs, if the system is idle (except for the
activity of the 'untrusted' tasks) you suspend normally.

b) you can't use cgroup for other purposes anymore. I.e. if you want to
have 2 groups that each only have half of the memory available, how
would you then integrate the cgroup-ignore-for-idle-approach with this?

two answers to this

1. does this matter? do you really need to combine this 'suspend, even if
there are processes trying to run' with other cgroup limitations?

2. who says that this must be limited to one cgroup? a cgroup can have
several different flags/limits set on it, so why can't one of them be this
'ignore for suspend' flag?

these seem like simple issues, what I don't know is if it's possible for
the process that controlls suspend to follow such a flag without major
surgury on it's innards (if it can, this seems like a easy win, but I can
imagine internal designs where the software just knows that _something_ is
trying to run and would have a very hard time figuring out what)

David Lang

Well, i fear it becomes some sort of parallel-tree structure...
If you want a cgroups-partitioning for one kind of attribute you now
need 2 containers for every possible stamping of that attribute... one
being flagged with 'ignore-for-suspend-decision' and one without that
Do you see what I'm getting at, or do I need more coffee and it is
irelevant to this concept?

yes, it could mean a doubleing in the number of cgroups that you need on a system. and if there are other features like this you can end up in a geometric explosion in the number of cgroups.

in practice I question if there is likely to be a need for this sort of thing on a system that's complex enough to use cgroups for other purposes.

in particular, in this case the 'ignore for syspend' flag is not going to be set for programs that are trusted to be well behaved. Such programs are unlikely to be placed under other restraints (because they _are_ trusted to be well behaved)

David Lang
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