Re: [RFC PATCH v3 5/5] sched: activate active load balancing in new idle cpus

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2008-11-11 at 22:34 +0530, Vaidyanathan Srinivasan wrote:
> * Peter Zijlstra <a.p.zijlstra@chello.nl> [2008-11-11 14:47:15]:
> 
> > On Tue, 2008-11-11 at 00:03 +0530, Vaidyanathan Srinivasan wrote:
> > > Active load balancing is a process by which migration thread
> > > is woken up on the target CPU in order to pull current
> > > running task on another package into this newly idle
> > > package.
> > > 
> > > This method is already in use with normal load_balance(),
> > > this patch introduces this method to new idle cpus when
> > > sched_mc is set to POWERSAVINGS_BALANCE_WAKEUP.
> > > 
> > > This logic provides effective consolidation of short running
> > > daemon jobs in a almost idle system
> > > 
> > > The side effect of this patch may be ping-ponging of tasks
> > > if the system is moderately utilised. May need to adjust the
> > > iterations before triggering.
> > 
> > OK, I'm so not getting this patch..
> > 
> > if normal newly idle balancing fails that means the other runqueue has
> > only a single task on it (or some other really stubborn stuff), so then
> > you go move that one task that is already running, from one cpu to
> > another.
> > 
> > _why_?
> > 
> > The only answer I can come up with is that you prefer one cpu's
> > idle-ness over another - which makes sense, as you try to get whole
> > packages idle.
> 
> Your answer is correct.  We want to move that one task from a non-idle
> cpu to this cpu that is just going to be idle.  
> 
> This is the same method used to move task in load_balance(), I have
> extended it for load_balance_newidle() to make the consolidation
> faster at sched_mc=2.
> 
> 
> > But I'm not seeing where that package logic is hidden..
> 
> 
> The package logic comes from find_busiest_group().  If there are no
> imbalance, then find_busiest_group() will return NULL.  However when
> sched_mc={1,2} then find_busiest_group() will select a group
> from which a running task may be pulled to this cpu in order to make
> the other package idle.  If there is no opportunity to make a package
> idle and if there are no imbalance, then find_busiest_group() will
> return NULL and no action will be taken in load_balance_newidle().
> 
> Under normal task pull operation due to imbalance, there will be more
> than one task in the source run queue and move_tasks() will succeed.
> ld_moved will be true and the active balance code will not be
> triggered.
> 
> If we enter a scenario where we are moving the only running task from
> another cpu, then this should have been suggested by
> find_busiest_group's sched_mc balance logic and thus moving that task
> will potentially freeup the source package.
> 
> Thanks for the careful review.

Ah, right, thanks!

Could you clarify this by adding a comment to this effect right before
the added code?