lkml.org 
[lkml]   [2013]   [Oct]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH V2 2/2] sched: Remove un-necessary iteration over sched domains to update nr_busy_cpus
Hi Kamalesh,

On 10/30/2013 02:53 PM, Kamalesh Babulal wrote:
> 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.

Right and it *should* return 0. There is no sibling domain that can
offload tasks from it. Therefore there is no point kicking nohz idle
balance.

Regards
Preeti U Murthy
>
> Thanks,
> Kamalesh.
>



\
 
 \ /
  Last update: 2013-10-30 11:21    [W:0.055 / U:3.432 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site