Re: [PATCH V2 2/2] sched: Remove un-necessary iteration over sched domains to update nr_busy_cpus

[Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>]

Hi Preeti,

> nr_busy_cpus parameter is used by nohz_kick_needed() to find out the number
> of busy cpus in a sched domain which has SD_SHARE_PKG_RESOURCES flag set.
> Therefore instead of updating nr_busy_cpus at every level of sched domain,
> since it is irrelevant, we can update this parameter only at the parent
> domain of the sd which has this flag set. Introduce a per-cpu parameter
> sd_busy which represents this parent domain.
> 
> In nohz_kick_needed() we directly query the nr_busy_cpus parameter
> associated with the groups of sd_busy.
> 
> By associating sd_busy with the highest domain which has
> SD_SHARE_PKG_RESOURCES flag set, we cover all lower level domains which could
> have this flag set and trigger nohz_idle_balancing if any of the levels have
> more than one busy cpu.
> 
> sd_busy is irrelevant for asymmetric load balancing. However sd_asym has been
> introduced to represent the highest sched domain which has SD_ASYM_PACKING flag set
> so that it can be queried directly when required.
> 
> While we are at it, we might as well change the nohz_idle parameter to be
> updated at the sd_busy domain level alone and not the base domain level of a CPU.
> This will unify the concept of busy cpus at just one level of sched domain
> where it is currently used.
> 
> Signed-off-by: Preeti U Murthy<preeti@linux.vnet.ibm.com>
> ---
>  kernel/sched/core.c  |    6 ++++++
>  kernel/sched/fair.c  |   38 ++++++++++++++++++++------------------
>  kernel/sched/sched.h |    2 ++
>  3 files changed, 28 insertions(+), 18 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c06b8d3..e6a6244 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5271,6 +5271,8 @@ DEFINE_PER_CPU(struct sched_domain *, sd_llc);
>  DEFINE_PER_CPU(int, sd_llc_size);
>  DEFINE_PER_CPU(int, sd_llc_id);
>  DEFINE_PER_CPU(struct sched_domain *, sd_numa);
> +DEFINE_PER_CPU(struct sched_domain *, sd_busy);
> +DEFINE_PER_CPU(struct sched_domain *, sd_asym);
> 
>  static void update_top_cache_domain(int cpu)
>  {
> @@ -5282,6 +5284,7 @@ static void update_top_cache_domain(int cpu)
>  	if (sd) {
>  		id = cpumask_first(sched_domain_span(sd));
>  		size = cpumask_weight(sched_domain_span(sd));
> +		rcu_assign_pointer(per_cpu(sd_busy, cpu), sd->parent);
>  	}


consider a machine with single socket, dual core with HT enabled. The top most
domain is also the highest domain with SD_SHARE_PKG_RESOURCES flag set,
i.e MC domain (the machine toplogy consist of SIBLING and MC domain).

# lstopo-no-graphics --no-bridges --no-io
Machine (7869MB) + Socket L#0 + L3 L#0 (3072KB)
  L2 L#0 (256KB) + L1d L#0 (32KB) + L1i L#0 (32KB) + Core L#0
    PU L#0 (P#0)
    PU L#1 (P#1)
  L2 L#1 (256KB) + L1d L#1 (32KB) + L1i L#1 (32KB) + Core L#1
    PU L#2 (P#2)
    PU L#3 (P#3)

With this approach parent of MC domain is NULL and given that sd_busy is NULL,
nr_busy_cpus of sched domain sd_busy will never be incremented/decremented.
Resulting is nohz_kick_needed returning 0.

Thanks,
Kamalesh.