[PATCH 35/41] sched: Update rq clock before idle balancing

[Michael Wang]

On 05/01/2012 07:55 AM, Frederic Weisbecker wrote:

> idle_balance() is called from schedule() right before we schedule the
> idle task. It needs to record the idle timestamp at that time and for
> this the rq clock must be accurate. If the CPU is running tickless
> we need to update the rq clock manually.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec at gmail.com>
> Cc: Alessio Igor Bogani <abogani at kernel.org>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Avi Kivity <avi at redhat.com>
> Cc: Chris Metcalf <cmetcalf at tilera.com>
> Cc: Christoph Lameter <cl at linux.com>
> Cc: Daniel Lezcano <daniel.lezcano at linaro.org>
> Cc: Geoff Levand <geoff at infradead.org>
> Cc: Gilad Ben Yossef <gilad at benyossef.com>
> Cc: Hakan Akkan <hakanakkan at gmail.com>
> Cc: Ingo Molnar <mingo at kernel.org>
> Cc: Kevin Hilman <khilman at ti.com>
> Cc: Max Krasnyansky <maxk at qualcomm.com>
> Cc: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Stephen Hemminger <shemminger at vyatta.com>
> Cc: Steven Rostedt <rostedt at goodmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation at gmail.com>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> ---
>  kernel/sched/fair.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index eff80e0..cd871e7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4638,6 +4638,7 @@ void idle_balance(int this_cpu, struct rq *this_rq)
>  	int pulled_task = 0;
>  	unsigned long next_balance = jiffies + HZ;
> 
> +	update_nohz_rq_clock(this_rq);


I'm not sure but why we have to care nohz? if we really need an accurate
clock, we should do the update anyway, don't we?

Some thing also confused me is the description:
"If the CPU is running tickless we need to update the rq clock manually."

I think the cpu will enter tickless mode only when the idle thread
already switched in, then invoke
tick_nohz_idle_enter->tick_nohz_stop_sched_tick, isn't it?

And if we invoke idle_balance for a cpu, that means it hasn't enter
idle(current task is not idle task), so how can such a cpu in tickless mode?

Regards,
Michael Wang

>  	this_rq->idle_stamp = this_rq->clock;
> 
>  	if (this_rq->avg_idle < sysctl_sched_migration_cost)