Hi Jason,
On 03/31/2015 12:25 AM, Jason Low wrote:
> Hi Preeti,
>
> I noticed that another commit 4a725627f21d converted the check in
> nohz_kick_needed() from idle_cpu() to rq->idle_balance, causing a
> potentially outdated value to be used if this cpu is able to pull tasks
> using rebalance_domains(), and nohz_kick_needed() directly returning
> false.
I see that rebalance_domains() will be run at the end of the scheduler
tick interrupt handling. trigger_load_balance() only sets the softirq,
it does not call rebalance_domains() immediately. So the call graph
would be:
rq->idle_balance = idle_cpu()
|____trigger_load_balance()
|_____raise SCHED_SOFTIRQ - we are handling interrupt,hence defer
|____nohz_kick_needed()
|____rebalance_domains() run through the softirqd.
Correct me if I am wrong but since we do not pull any load between the
rq->idle_balance update and nohz_kick_needed(), we are safe in reading
rq->idle_balance in nohz_kick_needed().
>
> Would this patch also help address some of the issue you are seeing?
>
> Signed-off-by: Jason Low <jason.l...@hp.com>
> ---
> kernel/sched/fair.c | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fdae26e..ba8ec1a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7644,7 +7644,7 @@ 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;
Why is this hunk needed?
Regards
Preeti U Murthy
>
> rq = cpu_rq(balance_cpu);
>
> @@ -7687,7 +7687,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
> int nr_busy, cpu = rq->cpu;
> bool kick = false;
>
> - if (unlikely(rq->idle_balance))
> + if (unlikely(idle_cpu(cpu)))
> return false;
>
> /*
>
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