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    SubjectRe: [PATCH 3/7] idle, thermal, acpi: Remove home grown idle implementations
    On Thu, Nov 21, 2013 at 05:29:56PM +0100, Peter Zijlstra wrote:
    > On Thu, Nov 21, 2013 at 08:07:16AM -0800, Paul E. McKenney wrote:
    > > On Thu, Nov 21, 2013 at 09:21:51AM +0100, Peter Zijlstra wrote:
    > > > On Wed, Nov 20, 2013 at 04:54:06PM -0800, Jacob Pan wrote:
    > > > > On Wed, 20 Nov 2013 17:04:53 +0100
    > > > > Peter Zijlstra <peterz@infradead.org> wrote:
    > > > >
    > > > > > People are starting to grow their own idle implementations in various
    > > > > > disgusting ways. Collapse the lot and use the generic idle code to
    > > > > > provide a proper idle cycle implementation.
    > > > > >
    > > > > +Paul
    > > > >
    > > > > RCU and others rely on is_idle_task() might be broken with the
    > > > > consolidated idle code since caller of do_idle may have pid != 0.
    > > > >
    > > > > Should we use TS_POLL or introduce a new flag to identify idle task?
    > > >
    > > > PF_IDLE would be my preference, I checked and we seem to have a grand
    > > > total of 2 unused task_struct::flags left ;-)
    > >
    > > As long as RCU has some reliable way to identify an idle task, I am
    > > good. But I have to ask -- why can't idle injection coordinate with
    > > the existing idle tasks rather than temporarily making alternative
    > > idle tasks?
    >
    > Because that'd completely wreck how the scheduler selects tasks for just
    > these 2 arguably insane drivers.
    >
    > We'd have to somehow teach it to pick the actual idle task instead of
    > this one task, but keep scheduling the rest of the tasks like normal --
    > we very much should keep higher priority tasks running like normal.
    >
    > And we'd need a way to make it stop doing this 'proxy' execution.
    >
    > That said, once we manage to replace the entire PI implementation with a
    > proper proxy execution scheme, the above would be possible by having a
    > resource (rt_mutex) associated with every idle task, and always held by
    > that task.
    >
    > At that point we can do something like:
    >
    > rt_mutex_lock_timeout(cpu_idle_lock(cpu), jiffies);
    >
    > And get the idle thread executing in our stead.
    >
    > That said, idle is _special_ and I'd not be surprised we'd find a few
    > 'funnies' along the way of trying to get that to actually work.
    >
    > For now I'd rather not go there quite yet.

    Fair enough!

    Thanx, Paul



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