On Wed, 2015-04-01 at 22:59 -0700, Jason Low wrote:
> On Wed, 2015-04-01 at 18:04 +0100, Morten Rasmussen wrote:
> > On Wed, Apr 01, 2015 at 07:49:56AM +0100, Preeti U Murthy wrote:
>
> > > I am sorry I don't quite get this. Can you please elaborate?
> >
> > I think the scenario is that we are in nohz_idle_balance() and decide to
> > bail out because we have pulled some tasks, but before leaving
> > nohz_idle_balance() we want to check if more balancing is necessary
> > using nohz_kick_needed() and potentially kick somebody to continue.
>
> Also, below is an example patch.
>
> (Without the conversion to idle_cpu(), the check for rq->idle_balance
> would not be accurate anymore)
>
> ---
> kernel/sched/fair.c | 17 ++++++++++-------
> 1 files changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fdae26e..7749a14 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7620,6 +7620,8 @@ out:
> }
>
> #ifdef CONFIG_NO_HZ_COMMON
> +static inline bool nohz_kick_needed(struct rq *rq);
> +
> /*
> * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
> * rebalancing for all the cpus for whom scheduler ticks are stopped.
> @@ -7629,6 +7631,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum
> cpu_idle_type idle)
> int this_cpu = this_rq->cpu;
> struct rq *rq;
> int balance_cpu;
> + bool done_balancing = false;
>
> if (idle != CPU_IDLE ||
> !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> @@ -7644,7 +7647,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum
> cpu_idle_type idle)
> * balancing owner will pick it up.
> */
> if (need_resched())
> - break;
> + goto end;
>
> rq = cpu_rq(balance_cpu);
>
> @@ -7663,9 +7666,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum
> cpu_idle_type idle)
> if (time_after(this_rq->next_balance, rq->next_balance))
> this_rq->next_balance = rq->next_balance;
> }
> + done_balancing = true;
> nohz.next_balance = this_rq->next_balance;
> end:
> clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> + if (!done_balancing && nohz_kick_needed(this_rq))
> + nohz_balancer_kick();
> }
>
>
I think we can get rid of the done_balancing boolean
and make it a bit easier to read if we change the above code to
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bcfe320..08317dc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7557,8 +7557,13 @@ static void nohz_idle_balance(struct rq *this_rq, enum
cpu_idle_type idle)
* work being done for other cpus. Next load
* balancing owner will pick it up.
*/
- if (need_resched())
- break;
+ if (need_resched()) {
+ /* preparing to bail, kicking other cpu to continue */
+ clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
+ if (nohz_kick_needed(this_rq))
+ nohz_balance_kick();
+ return;
+ }
rq = cpu_rq(balance_cpu);
Thanks.
Tim
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