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

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Date:   Tue, 10 Apr 2018 18:07:17 +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

Hi Thomas,

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.

> 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.

> 
> Thanks,
> 
> 	tglx