linux-kernel - Re: [PATCH] time: tick-sched: use bool for tick

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Date:   Tue, 10 Apr 2018 22:08:07 +0800
From:   yuankuiz@...eaurora.org
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pm-owner@...r.kernel.org
Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped

On 2018-04-10 07:06 PM, Thomas Gleixner wrote:
> On Tue, 10 Apr 2018, yuankuiz@...eaurora.org wrote:
>> On 2018-04-10 05:10 PM, Thomas Gleixner wrote:
>> > On Tue, 10 Apr 2018, yuankuiz@...eaurora.org wrote:
>> > > On 2018-04-10 04:00 PM, Rafael J. Wysocki wrote:
>> > > > On Tue, Apr 10, 2018 at 9:33 AM,  <yuankuiz@...eaurora.org> wrote:
>> > > > > From: John Zhao <yuankuiz@...eaurora.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@...eaurora.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():
                         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)

> 
> Thanks,
> 
> 	tglx