Re: [PATCH 1/3] sched: enable interrupts and drop rq-lockduringnewidle balancing

["Gregory Haskins" <ghaskins@novell.com>]

>>> On Tue, Jun 24, 2008 at  8:24 AM, in message <1214310299.4351.27.camel@twins>,
Peter Zijlstra <peterz@infradead.org> wrote: 
> On Tue, 2008-06-24 at 07:15 -0600, Gregory Haskins wrote:
>> >>> On Tue, Jun 24, 2008 at  6:13 AM, in message 
> <1214302405.4351.21.camel@twins>,
>> Peter Zijlstra <peterz@infradead.org> wrote: 
>> > On Mon, 2008-06-23 at 17:04 -0600, Gregory Haskins wrote:
>> >> We do find_busiest_groups() et. al. without locks held for normal 
> balancing,
>> >> so lets do it for newidle as well.  It will allow other cpus to make
>> >> forward progress (against our RQ) while we try to balance and allow 
>> >> some interrupts to occur.
>> > 
>> > Is running f_b_g really that expensive? 
>> 
>> According to our oprofile data, yes.  I speculate that it works out that way 
> because most newidle
>> attempts result in "no imbalance".  But we were spending ~60%+ time in 
> find_busiest_groups()
>> because of all the heavy-context switching that goes on in PREEMPT_RT.  So 
> while f_b_g() is
>> probably cheaper than double-lock/move_tasks(), the ratio of occurrence is 
> off the charts in
>> comparison. Prior to this patch, those occurrences were 
> preempt-disabled/irq-disabled/rq->lock critical
>> sections.
>> 
>> So while it is not clear if f_b_g() is the actual cost, it is a convenient 
> (and legal, afaict) place to
>> deterministically reduce the rq->lock scope.  Additionally, doing so 
> measurably helps
>> performance, so I think its a win.  Without this patch you have to hope the 
> double_lock releases
>> this_rq, and even so were not checking for the NEEDS_RESCHED. 
> 
> See, having had this information in the changelog to begin with would
> have helped ;-)

What?  You can't read my mind?  :)

Good point, Peter.  Will fix on next drop.

-Greg

> 
> 
> 
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