Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with sched_domain_shared

[Matt Fleming <matt@codeblueprint.co.uk>]

On Mon, 09 May, at 12:48:11PM, Peter Zijlstra wrote:
> Move the nr_busy_cpus thing from its hacky sd->parent->groups->sgc
> location into the much more natural sched_domain_shared location.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/sched.h    |    1 +
>  kernel/sched/core.c      |   10 +++++-----
>  kernel/sched/fair.c      |   22 ++++++++++++----------
>  kernel/sched/sched.h     |    6 +-----
>  kernel/time/tick-sched.c |   10 +++++-----
>  5 files changed, 24 insertions(+), 25 deletions(-)
> 
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1059,6 +1059,7 @@ struct sched_group;
>  
>  struct sched_domain_shared {
>  	atomic_t	ref;
> +	atomic_t	nr_busy_cpus;
>  };
>  
>  struct sched_domain {

[...]

> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7842,13 +7842,13 @@ static inline void set_cpu_sd_state_busy
>  	int cpu = smp_processor_id();
>  
>  	rcu_read_lock();
> -	sd = rcu_dereference(per_cpu(sd_busy, cpu));
> +	sd = rcu_dereference(per_cpu(sd_llc, cpu));
>  
>  	if (!sd || !sd->nohz_idle)
>  		goto unlock;
>  	sd->nohz_idle = 0;
>  
> -	atomic_inc(&sd->groups->sgc->nr_busy_cpus);
> +	atomic_inc(&sd->shared->nr_busy_cpus);
>  unlock:
>  	rcu_read_unlock();
>  }

This breaks my POWER7 box which presumably doesn't have SD_SHARE_PKG_RESOURCES,

  NIP [c00000000012de68] .set_cpu_sd_state_idle+0x58/0x80
  LR [c00000000017ded4] .tick_nohz_idle_enter+0x24/0x90
  Call Trace:
  [c0000007774b7cf0] [c0000007774b4080] 0xc0000007774b4080 (unreliable)
  [c0000007774b7d60] [c00000000017ded4] .tick_nohz_idle_enter+0x24/0x90
  [c0000007774b7dd0] [c000000000137200] .cpu_startup_entry+0xe0/0x440
  [c0000007774b7ee0] [c00000000004739c] .start_secondary+0x35c/0x3a0
  [c0000007774b7f90] [c000000000008bfc] start_secondary_prolog+0x10/0x14

The following fixes it,

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 978b3ef2d87e..d27153adee4d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7920,7 +7920,8 @@ static inline void set_cpu_sd_state_busy(void)
 		goto unlock;
 	sd->nohz_idle = 0;
 
-	atomic_inc(&sd->shared->nr_busy_cpus);
+	if (sd->shared)
+		atomic_inc(&sd->shared->nr_busy_cpus);
 unlock:
 	rcu_read_unlock();
 }
@@ -7937,7 +7938,8 @@ void set_cpu_sd_state_idle(void)
 		goto unlock;
 	sd->nohz_idle = 1;
 
-	atomic_dec(&sd->shared->nr_busy_cpus);
+	if (sd->shared)
+		atomic_dec(&sd->shared->nr_busy_cpus);
 unlock:
 	rcu_read_unlock();
 }