linux-kernel - Re: [PATCH 2/3] sched: introduce per-cpu var next

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Date:   Tue, 24 Apr 2018 15:39:40 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        daniel.lezcano@...aro.org, steven.sistare@...cle.com,
        dhaval.giani@...cle.com, rohit.k.jain@...cle.com
Subject: Re: [PATCH 2/3] sched: introduce per-cpu var next_cpu to track search
 limit



On 04/24/2018 05:47 AM, Peter Zijlstra wrote:
> On Mon, Apr 23, 2018 at 05:41:15PM -0700, subhra mazumdar wrote:
>> @@ -17,6 +17,7 @@
>>   #include <trace/events/sched.h>
>>   
>>   DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
>> +DEFINE_PER_CPU_SHARED_ALIGNED(int, next_cpu);
>>   
>>   #if defined(CONFIG_SCHED_DEBUG) && defined(HAVE_JUMP_LABEL)
>>   /*
>> @@ -6018,6 +6019,7 @@ void __init sched_init(void)
>>   		struct rq *rq;
>>   
>>   		rq = cpu_rq(i);
>> +		per_cpu(next_cpu, i) = -1;
> If you leave it uninitialized it'll be 0, and we can avoid that extra
> branch in the next patch, no?
0 can be a valid cpu id. I wanted to distinguish the first time. The branch
predictor will be fully trained so will not have any cost.