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Date:   Wed, 11 Apr 2018 07:20:55 +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 07:09 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?
>>> 
>>>> 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