linux-kernel - Re: [PATCH 1/5] sched: limit cpu search in select_idle

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Date:   Tue, 12 Jun 2018 15:12:50 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     kbuild test robot <lkp@...el.com>
Cc:     kbuild-all@...org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, mingo@...hat.com, steven.sistare@...cle.com,
        dhaval.giani@...cle.com, rohit.k.jain@...cle.com,
        daniel.lezcano@...aro.org
Subject: Re: [PATCH 1/5] sched: limit cpu search in select_idle_cpu



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();