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 <[email protected]>
> Cc: Alessio Igor Bogani <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Avi Kivity <[email protected]>
> Cc: Chris Metcalf <[email protected]>
> Cc: Christoph Lameter <[email protected]>
> Cc: Daniel Lezcano <[email protected]>
> Cc: Geoff Levand <[email protected]>
> Cc: Gilad Ben Yossef <[email protected]>
> Cc: Hakan Akkan <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Kevin Hilman <[email protected]>
> Cc: Max Krasnyansky <[email protected]>
> Cc: Paul E. McKenney <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Stephen Hemminger <[email protected]>
> Cc: Steven Rostedt <[email protected]>
> Cc: Sven-Thorsten Dietrich <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> ---
> 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)
On Wed, May 02, 2012 at 11:36:07AM +0800, Michael Wang wrote:
> 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 <[email protected]>
> > Cc: Alessio Igor Bogani <[email protected]>
> > Cc: Andrew Morton <[email protected]>
> > Cc: Avi Kivity <[email protected]>
> > Cc: Chris Metcalf <[email protected]>
> > Cc: Christoph Lameter <[email protected]>
> > Cc: Daniel Lezcano <[email protected]>
> > Cc: Geoff Levand <[email protected]>
> > Cc: Gilad Ben Yossef <[email protected]>
> > Cc: Hakan Akkan <[email protected]>
> > Cc: Ingo Molnar <[email protected]>
> > Cc: Kevin Hilman <[email protected]>
> > Cc: Max Krasnyansky <[email protected]>
> > Cc: Paul E. McKenney <[email protected]>
> > Cc: Peter Zijlstra <[email protected]>
> > Cc: Stephen Hemminger <[email protected]>
> > Cc: Steven Rostedt <[email protected]>
> > Cc: Sven-Thorsten Dietrich <[email protected]>
> > Cc: Thomas Gleixner <[email protected]>
> > ---
> > 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?
This concerns adaptive tickless CPUs only. So I wanted to keep the overhead
low for CPUs that are not in adaptive tickless mode. update_nohz_rq_clock()
takes care of that. It only updates the rq clock if the CPU is adaptive tickless.
>
> 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?
An adaptive tickless CPU tries to shutdown the tick even when the CPU
is not idle. By the time we are about to sleep and schedule the idle
task, we may be already tickless for a while.
>
> 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)
>
> --
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