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

[yuankuiz]

Typo...

On 2018-04-10 10:08 PM, yuankuiz@xxxxxxxxxxxxxx wrote:
> On 2018-04-10 07:06 PM, Thomas Gleixner wrote:
> > On Tue, 10 Apr 2018, yuankuiz@xxxxxxxxxxxxxx wrote:
> > > On 2018-04-10 05:10 PM, Thomas Gleixner wrote:
> > > > On Tue, 10 Apr 2018, yuankuiz@xxxxxxxxxxxxxx wrote:
> > > > > On 2018-04-10 04:00 PM, Rafael J. Wysocki wrote:
> > > > > > On Tue, Apr 10, 2018 at 9:33 AM,  <yuankuiz@xxxxxxxxxxxxxx> wrote:
> > > > > > > From: John Zhao <yuankuiz@xxxxxxxxxxxxxx>
> > > > > > >
> > > > > > > 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@xxxxxxxxxxxxxx>
> > > > > > > ---
> > > > > > >  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