Re: [PATCH 3/7] idle, thermal, acpi: Remove home grown idle implementations

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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