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Subject: Re: [PATCH 1/5] sched: limit cpu search in select_idle_cpu
To:     kbuild test robot <lkp@intel.com>
Cc:     kbuild-all@01.org, linux-kernel@vger.kernel.org,
        peterz@infradead.org, mingo@redhat.com, steven.sistare@oracle.com,
        dhaval.giani@oracle.com, rohit.k.jain@oracle.com,
        daniel.lezcano@linaro.org
References: <20180612175002.31453-2-subhra.mazumdar@oracle.com>
 <201806130240.AQdYDV0h%fengguang.wu@intel.com>
From:   Subhra Mazumdar <subhra.mazumdar@oracle.com>
Message-ID: <e128778b-c079-700b-ffad-4749e66d3bbb@oracle.com>
Date:   Tue, 12 Jun 2018 15:12:50 -0700
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On 06/12/2018 01:33 PM, kbuild test robot wrote:
> Hi subhra,
>
> Thank you for the patch! Perhaps something to improve:
>
> [auto build test WARNING on tip/sched/core]
> [also build test WARNING on v4.17 next-20180612]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>
> url:    https://github.com/0day-ci/linux/commits/subhra-mazumdar/Improve-scheduler-scalability-for-fast-path/20180613-015158
> config: i386-randconfig-x070-201823 (attached as .config)
> compiler: gcc-7 (Debian 7.3.0-16) 7.3.0
> reproduce:
>          # save the attached .config to linux build tree
>          make ARCH=i386
>
> All warnings (new ones prefixed by >>):
>
>     kernel/sched/fair.c: In function 'select_idle_cpu':
>>> kernel/sched/fair.c:6396:3: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
>        u64 span_avg = sd->span_weight * avg_idle;
>        ^~~
>
>
<snip>

I fixed this patch, please try the following

---8<---

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e497c05..7243146 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6372,7 +6372,7 @@ static int select_idle_cpu(struct task_struct *p, 
struct sched_domain *sd, int t
 ??????? u64 avg_cost, avg_idle;
 ??????? u64 time, cost;
 ??????? s64 delta;
-?????? int cpu, nr = INT_MAX;
+?????? int cpu, limit, floor, nr = INT_MAX;

 ??????? this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
 ??????? if (!this_sd)
@@ -6390,10 +6390,17 @@ static int select_idle_cpu(struct task_struct 
*p, struct sched_domain *sd, int t

 ??????? if (sched_feat(SIS_PROP)) {
 ??????????????? u64 span_avg = sd->span_weight * avg_idle;
-?????????????? if (span_avg > 4*avg_cost)
+?????????????? floor = cpumask_weight(topology_sibling_cpumask(target));
+?????????????? if (floor < 2)
+?????????????????????? floor = 2;
+?????????????? limit = 2*floor;
+?????????????? if (span_avg > floor*avg_cost) {
 ??????????????????????? nr = div_u64(span_avg, avg_cost);
-?????????????? else
-?????????????????????? nr = 4;
+?????????????????????? if (nr > limit)
+?????????????????????????????? nr = limit;
+?????????????? } else {
+?????????????????????? nr = floor;
+?????????????? }
 ??????? }

 ??????? time = local_clock();