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Date:   Fri, 20 Apr 2018 09:47:05 +0800
From:   yuankuiz@codeaurora.org
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Len Brown <len.brown@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-pm-owner@vger.kernel.org, akpm@linux-foundation.org,
        joe@perches.com
Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped
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On 2018-04-11 07:20 AM, yuankuiz@codeaurora.org wrote:
> ++
> On 2018-04-11 07:09 AM, yuankuiz@codeaurora.org wrote:
>> ++
>> 
>> On 2018-04-10 10:49 PM, yuankuiz@codeaurora.org wrote:
>>> Typo...
>>> 
>>> On 2018-04-10 10:08 PM, yuankuiz@codeaurora.org wrote:
>>>> On 2018-04-10 07:06 PM, Thomas Gleixner wrote:
>>>>> On Tue, 10 Apr 2018, yuankuiz@codeaurora.org wrote:
>>>>>> On 2018-04-10 05:10 PM, Thomas Gleixner wrote:
>>>>>> > On Tue, 10 Apr 2018, yuankuiz@codeaurora.org wrote:
>>>>>> > > On 2018-04-10 04:00 PM, Rafael J. Wysocki wrote:
>>>>>> > > > On Tue, Apr 10, 2018 at 9:33 AM,  <yuankuiz@codeaurora.org> wrote:
>>>>>> > > > > From: John Zhao <yuankuiz@codeaurora.org>
>>>>>> > > > >
>>>>>> > > > > Variable tick_stopped returned by tick_nohz_tick_stopped
>>>>>> > > > > can have only true / false values. Since the return type
>>>>>> > > > > of the tick_nohz_tick_stopped is also bool, variable
>>>>>> > > > > tick_stopped nice to have data type as bool in place of unsigned int.
>>>>>> > > > > Moreover, the executed instructions cost could be minimal
>>>>>> > > > > without potiential data type conversion.
>>>>>> > > > >
>>>>>> > > > > Signed-off-by: John Zhao <yuankuiz@codeaurora.org>
>>>>>> > > > > ---
>>>>>> > > > >  kernel/time/tick-sched.h | 2 +-
>>>>>> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>> > > > >
>>>>>> > > > > diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
>>>>>> > > > > index 6de959a..4d34309 100644
>>>>>> > > > > --- a/kernel/time/tick-sched.h
>>>>>> > > > > +++ b/kernel/time/tick-sched.h
>>>>>> > > > > @@ -48,8 +48,8 @@ struct tick_sched {
>>>>>> > > > >         unsigned long                   check_clocks;
>>>>>> > > > >         enum tick_nohz_mode             nohz_mode;
>>>>>> > > > >
>>>>>> > > > > +       bool                            tick_stopped    : 1;
>>>>>> > > > >         unsigned int                    inidle          : 1;
>>>>>> > > > > -       unsigned int                    tick_stopped    : 1;
>>>>>> > > > >         unsigned int                    idle_active     : 1;
>>>>>> > > > >         unsigned int                    do_timer_last   : 1;
>>>>>> > > > >         unsigned int                    got_idle_tick   : 1;
>>>>>> > > >
>>>>>> > > > I don't think this is a good idea at all.
>>>>>> > > >
>>>>>> > > > Please see https://lkml.org/lkml/2017/11/21/384 for example.
>>>>>> > > [ZJ] Thanks for this sharing. Looks like, this patch fall into the case of
>>>>>> > > "Maybe".
>>>>>> >
>>>>>> > This patch falls into the case 'pointless' because it adds extra storage
>>>>>> [ZJ] 1 bit vs 1 bit. no more.
>>>>> 
>>>>> Groan. No. Care to look at the data structure? You create a new 
>>>>> storage,
>>>> [ZJ] Say, {unsigned int, unsigned int, unsigned int, unsigned int,
>>>> unsigned int} becomes
>>>>           {bool        , unsigned int, unsigned int, unsigned int, 
>>>> unsigned int}
>>>> As specified by the rule No.10 at the section 6.7.2.1 of C99 TC2 as:
>>>> "If enough space remains, a bit-field that immediately follows 
>>>> another
>>>> bit-field in a
>>>> structure shall be packed into adjacent bits of the same unit." What
>>>> is the new storage so far?
[ZJ] Further prototyping has been given based on gcc for both of x86_64 
and armv8-a,
      unsigned int and bool share the same 4 bytes without the addtional 
storage for sure.
      Open this and welcome if any other difference behaviour could be 
captured.
>>>> 
>>>>> which is incidentally merged into the other bitfield by the 
>>>>> compiler at a
>>>>> different bit position, but there is no guarantee that a compiler 
>>>>> does
>>>>> that. It's free to use distinct storage for that bool based bit.
>>>> [ZJ] Per the rule No.10 at section 6.7.2.1 of C99 TC2 as:
>>>> " If insufficient space remains, whether  a  bit-field  that  does
>>>> not  fit  is  put  into
>>>> the  next  unit  or overlaps  adjacent  units  is 
>>>> implementation-defined."
>>>> So, implementation is never mind which type will be stored if any.
>>>> 
>>>>> >> > for no benefit at all.
>>>>>> [ZJ] tick_stopped is returned by the tick_nohz_tick_stopped() 
>>>>>> which is bool.
>>>>>> The benefit is no any potiential type conversion could be minded.
>>>>> 
>>>>> A bit stays a bit. 'bool foo : 1;' or 'unsigned int foo : 1' has to 
>>>>> be
>>>>> evaluated as a bit. So there is a type conversion from BIT to bool 
>>>>> required
>>>>> because BIT != bool.
>>>> [ZJ] Per the rule No.9 at section 6.7.2.1 of C99 TC2 as:
>>>> "If  the  value  0  or  1  is  stored  into  a  nonzero-width
>>>> bit-field  of  types
>>>> _Bool, the value of the bit-field shall compare equal to the value 
>>>> stored."
>>>> Obviously, it is nothing related to type conversion actually.
>>>>> 
>>>>> By chance the evaluation can be done by evaluating the byte in 
>>>>> which the
>>>>> bit is placed just because the compiler knows that the remaining 
>>>>> bits are
>>>>> not used. There is no guarantee that this is done, it happens to be 
>>>>> true
>>>>> for a particular compiler.
>>>> [ZJ] Actually, such as GCC owe that kind of guarantee to be promised 
>>>> by ABI.
>>>>> 
>>>>> But that does not make it any more interesting. It just makes the 
>>>>> code
>>>>> harder to read and eventually leads to bigger storage.
>>>> [ZJ] To get the benctifit to be profiled, it is given as:
>>>> number of instructions of function tick_nohz_tick_stopped():
>>> [ZJ] Here, I used is not the tick_nohz_tick_stopped(), but an 
>>> evaluation() as:
>>> #include <stdio.h>
>>> #include <stdbool.h>
>>> 
>>> struct tick_sched {
>>>         unsigned int inidle             : 1;
>>>         unsigned int tick_stopped       : 1;
>>> };
>>> 
>>> bool get_status()
>>> {
>>>         struct tick_sched *ts;
>>>         ts->tick_stopped = 1;
>>>         return ts->tick_stopped;
>>> }
>>> 
>>> int main()
>>> {
>>>         if (get_status()) return 0;
>>>         return 0;
>>> }
>>> 
>>> [ZJ] Toggle the declaration of tick_stopped in side of the tick_sched
>>> structure for comparison.
>>> 
>>> 
>>>>                         original: 17
>>>>                         patched:  14
>>>>      Which was saved is:
>>>>                movzbl	%al, %eax
>>>>                testl	%eax, %eax
>>>>                setne    %al
>>>>      Say, 3 / 17 = 17 % could be gained in the instruction executed
>>>> for this function can be evaluated.
>>>> 
>>>> Note:
>>>>      The environment I used is:
>>>>                OS : Ubuntu Desktop 16.04 LTS
>>>>                gcc: 6.3.0                       (without 
>>>> optimization
>>>> for in general purpose)
>>>> 
>>>>> 
>>> 
>>> Just FYI.
>>> 
>>> Thanks,
>>> ZJ