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Date:   Wed, 11 Apr 2018 07:09:28 +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
Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped
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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