linux-kernel - Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in check_preempt

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Date:   Thu, 19 Dec 2019 16:53:33 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] sched: Micro optimization in pick_next_task() and in
 check_preempt_curr()

On 19.12.2019 16:50, Steven Rostedt wrote:
> On Thu, 19 Dec 2019 15:39:14 +0300
> Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> 
>> @@ -6569,6 +6558,11 @@ void __init sched_init(void)
>>  	unsigned long ptr = 0;
>>  	int i;
>>  
>> +	BUG_ON(&idle_sched_class > &fair_sched_class ||
>> +		&fair_sched_class > &rt_sched_class ||
>> +		&rt_sched_class > &dl_sched_class ||
>> +		&dl_sched_class > &stop_sched_class);
>> +
> 
> Can this be a BUILD_BUG_ON? These address should all be constants.

BUILD_BUG_ON() is compile-time check, while address is assigned
at link time, isn't it?!

Anyway, plain BUILD_BUG_ON() fails here with the following:

In file included from ./arch/x86/include/asm/current.h:5,
                 from ./include/linux/sched.h:12,
                 from kernel/sched/sched.h:5,
                 from kernel/sched/core.c:9:
kernel/sched/core.c: In function ‘sched_init’:
./include/linux/compiler.h:394:38: error: call to ‘__compiletime_assert_6561’ declared with attribute error: BUILD_BUG_ON failed: &idle_sched_class > &fair_sched_class || &fair_sched_class > &rt_sched_class || &rt_sched_class > &dl_sched_class || &dl_sched_class > &stop_sched_class
  394 |  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
      |                                      ^
./include/linux/compiler.h:375:4: note: in definition of macro ‘__compiletime_assert’
  375 |    prefix ## suffix();    \
      |    ^~~~~~
./include/linux/compiler.h:394:2: note: in expansion of macro ‘_compiletime_assert’
  394 |  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
      |  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:50:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
   50 |  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |  ^~~~~~~~~~~~~~~~
kernel/sched/core.c:6561:2: note: in expansion of macro ‘BUILD_BUG_ON’
 6561 |  BUILD_BUG_ON(&idle_sched_class > &fair_sched_class ||
      |  ^~~~~~~~~~~~


> -- Steve
> 
> 
> 
>>  	wait_bit_init();
>>