Re: [PATCH 3/9] sched: Move idle_stamp up to the core

[Peter Zijlstra]

On Thu, Jan 23, 2014 at 03:39:36PM +0100, Daniel Lezcano wrote:
> On 01/23/2014 01:58 PM, Peter Zijlstra wrote:
> >On Tue, Jan 21, 2014 at 12:17:57PM +0100, Peter Zijlstra wrote:
> >>From: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> >>
> >>The idle_balance modifies the idle_stamp field of the rq, making this
> >>information to be shared across core.c and fair.c. As we can know if the
> >>cpu is going to idle or not with the previous patch, let's encapsulate the
> >>idle_stamp information in core.c by moving it up to the caller. The
> >>idle_balance function returns true in case a balancing occured and the cpu
> >>won't be idle, false if no balance happened and the cpu is going idle.
> >>
> >>Cc: alex.shi@xxxxxxxxxx
> >>Cc: peterz@xxxxxxxxxxxxx
> >>Cc: mingo@xxxxxxxxxx
> >>Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> >>Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >>Link: http://lkml.kernel.org/r/1389949444-14821-3-git-send-email-daniel.lezcano@xxxxxxxxxx
> >>---
> >>  kernel/sched/core.c  |    2 +-
> >>  kernel/sched/fair.c  |   14 ++++++--------
> >>  kernel/sched/sched.h |    2 +-
> >>  3 files changed, 8 insertions(+), 10 deletions(-)
> >>
> >>--- a/kernel/sched/core.c
> >>+++ b/kernel/sched/core.c
> >>@@ -2680,7 +2680,7 @@ static void __sched __schedule(void)
> >>  	pre_schedule(rq, prev);
> >>
> >>  	if (unlikely(!rq->nr_running))
> >>-		idle_balance(rq);
> >>+		rq->idle_stamp = idle_balance(rq) ? 0 : rq_clock(rq);
> >
> >OK, spotted a problem here..
> >
> >So previously idle_stamp was set _before_ actually doing idle_balance(),
> >and that was RIGHT, because that way we include the cost of actually
> >doing idle_balance() into the idle time.
> >
> >By not including the cost of idle_balance() you underestimate the 'idle'
> >time in that if idle_balance() filled the entire idle time we account 0
> >idle, even though we had 'plenty' of time to run the entire thing.
> >
> >This leads to not running idle_balance() even though we have the time
> >for it.
> 
> Good catch. How did you notice that ?

Staring at that code for too long :-)

> >So we very much want something like:
> >
> >
> >   if (!rq->nr_running)
> >   	rq->idle_stamp = rq_clock(rq);
> >
> >   p = pick_next_task(rq, prev);
> >
> >   if (!is_idle_task(p))
> >   	rq->idle_stamp = 0;
> 
> Is this code assuming idle_balance() is in pick_next_task ?

Yeah, I'm trying to make that work.
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